"No admittance without the password or passcard," warns a sign on the door of the Montgomery County school building.Inside an old classroom are stockbrokers and marketing representatives, administrators and management analysts, pastors and principals, all of them black.

The occasion is a Montgomery chapter meeting of Alpha Phi Alpha, the nation's oldest black fraternity, founded at predominantly white Cornell University in 1906 by seven students who felt alienated and excluded from campus life. On a cold autumn night 75 years later, the discussion starts with voter education strategies and quickly ranges over everything from layoffs of federal workers to human rights in South Africa.

But the primary concern of this prosperous, entirely male group is what its president calls the deteriorating racial climate in Montgomery County -- an ominous increase in hate group activity and recent setbacks for black children in its public schools.

"Racial epithets and KKK signs are emblazoned on the walls of school property," says Alpha President John Diggs, a deputy director of a branch of the National Institutes of Health. "Students are burning crosses by night. There is turmoil in the community. There is unrest in the schools."

For the Alpha "brothers," it is a picture that evokes too many memories of the past. As successful upper middle-class professionals, they know what it is to struggle in a white world, and now they have become a major force in what many blacks see as a fight to preserve their rights in an increasingly conservative county.

To members of the group, the signs are clear. Through October of this year, Montgomery County police received reports of 84 racially or religiously motivated incidents, ranging from threatening telephone calls and vandalism to cross burnings and assaults. In all of 1980, police received only 15 such reports.

In the county's public school system, an area of particular concern to the Alphas, they have watched the county school board abolish its Minority Relations Monitoring Committee in favor of a "less confrontational" group and alter its racial balance policy in a way that blacks say increases racial polarization in the schools.

Roscoe Nix, an Alpha member who serves as president of the Montgomery County NAACP, speaks for many of the county's 50,756 blacks when he says the racial situation was better in the late 70s than it is now. "From the standpoint of racial atmosphere," he says, "it was much better then than it is today. In the last three years we have undergone a period of regression."

In this atmosphere the Alpha chapter has emerged as the most influential and effective advocate for black interests in the county. Its 152 members are sophisticated as well as organized and have sought to overcome the disadvantage of being members of a group that, while increasing in numbers, currently makes up only 8.8 percent of the county and has no political tradition.

In the past, especially during the civil rights struggles of the 60s, the Alphas and other black Green letter organizations were criticized by more militant blacks as being bourgeois and snobbish. Others claimed that the fraternities themselves were exclusionary in their early years, limiting their membership to lighter-skinned blacks.

David Anderson, now a deputy director of social services for the Veterans Administration, was one who became disenchanted with the group in the '60s and joined the picked lines of the Congress of Racial Equality (CORE).

"I was influenced by the sit-ins and saw a need for a more active involvement," he said.

Later, however, he joined the Montgomery Alphas as a life member. "They had their act together," Anderson explained. "They were getting results and not wasting their total resources on inconsequential junk."

But success has not always come easily for the Alphas. The chapter was part of the losing battle to save a 45-hour black culture course once mandatory for all Montgomery school teachers, and its objected to the recent decision changing the racial balance of the county's public schools. The most recent setback was last summer's abolition of the school system's Minority Relations Monitoring Committee, which had been dominated by blacks.

But the fraternity, in concert with other minority organizations, succeeded in organizing a boycott of the school board's new committee and vowed to continue the work of the old group. Only three blacks eventually applied for the new committee, according to school board member Blair Ewing.

The chapter has also engaged in a number of successful fund-raising and education activities. Since 1973, it has handed out a total of $44,000 in $500 scholarships, raised money for the United Negro College Fund, the NAACP and the National Urban League, and sponsored health fairs in black neighborhoods, black heritage programs, and seminars on black business.

In supporting these programs, the Montgomery Alphas are following the tradition of black fraternal organizations, which in addition to their social aspects have been a vehicle for black middle-class involvement in social and political issues. In Washington, for example, Delta Sigma Theta, a black sorority, is currently sponsoring a $7 million federally funded apartment house for the elderly and handicapped.

Alpha Phi Alpha has counted as members many of the nation's most prominent blacks, including the Rev. Martin Luther King Jr., Supreme Court Justice Thurgod Marshall, mayor-elect of Atlanta Andrew Young, former Massachusetts Sen. Edward W. Brooke, as well as mayors Henry L. Marsh of Richmond and Marion Barry of Washington.

The Montgomery chapter has a similar profile of successful professionals, with the significant difference that they live in the suburbs.

Most of the Alphas are in their 40s and 50s, have two or three children, and live in homes that are now worth an average of $100,000. Many earn salaries of $40,000 to $45,000 per year and most often are employed with federal agencies and in the field of education, although the most recent trends show a shift to private sector jobs.

Only 30 percent are native to the Washington area. The vast majority of them live in the Wheaton-Silver Spring area of the county. Only a handful live in the more exclusive sections like Chevy Chase, Potomac and Bethesda.

"The overwhelming majority of black professionals in this county live east of Georgia Avenue," said past Alpha president Hanley J. Norment, director of the office of civil rights for the National Highway Transportation Safety Administration, citing what he called the pressure of real estate agents to maintain segregated housing in the 60s.

It was the fraternity's impressive record and the fact that so many national black leaders were members that drew most of the Montgomery Alphas to the group while they attended courses at black colleges in the south and at predominantly white universities in the east and midwest.

When they were in school, blacks were excluded from membership from most white fraternities and segregation was common.

John H. Harvey Sr., for example, attended Ohio State University from 1939 to 1942 when blacks were not allowed to live on campus. George Haley, the second black to graduate from the University of Arkansas law school, couldn't sit in the library. "It was a challenge," said the former Kansas legislator and brother of author Alex Haley. "But it was also frightening, to a certain extent."

It was an isolation that many of the local Alphas did not expect to find in Montgomery County. Most arrived in the mid '60s and early '70s when blacks made up less than 5 percent of the county's population.

Practically all were drawn to the Washington area because of job opportunities and most said they settled in Montgomery County because of the reputation of its schools and liberal government. Yet once they arrived they discovered that adjusting to life in white suburbia wasn't going to be easy.

James C. Moone, an Alpha and current president of the Montgomery County Black Coalition, moved to the county in 1966 and was hardly prepared for the response he received. Landlords openly told him they wouldn't rent to blacks, he said. But the fact that blacks in the county needed to band together was even more apparent when he finally purchased his Rockville home.

Moone said he returned home one evening to find that his three white neighbors had erected a 6 1/2 foot, solid wooden fence around his property. "It was a shock. We had come from the Atlanta area and they had signs down there that told us where not to go," Moone said. "Here, there were invisible signs."

An equally important reason for the founding of the chapter in 1970 was the need for a social oasis for black men in a county where there were few opportunities for blacks to socialize.

"One of the reasons we formed this was because blacks were so isolated. You would go into a shopping center, finally see another black face and just want to stop and talk to them," said Norment, who moved to the county in 1966 and described the social outlets for blacks then as "dismal."

Robert Hatchel, one of the principal founders of the Montgomery chapter, described why the group was started. "I felt that with our orientation there was a lot that we could do in Montgomery County. We just needed an organized group to rally around."

The passage of a fair housing law in the late 1960s reversed a demographic trend that had seen the black share of Montgomery County's population dwindle since the turn of the century.

In the next 10 years, the black population more than doubled. Nix, the current president of the county's NAACP, was elected to the school board; two more Alphas were appointed associate superintendents, and the Minority Relations Minitoring Committee was established by the school board as a watchdog for the rights of minority students.

In 1978, however, conservatives won control of the school board, setting off major changes beginning with the replacement of superintendent Charles Bernardo, who was well liked by minorities because of his support for affirmative action programs.

That election, many blacks believe, marked a change in the county's racial atmosphere. To the Alphas, it came as a surprise. Some blame it on a national trend, but others, like Norment, view it as an outgrowth of increasingly tough economic times.

"When the county's black population doubled in the last decade," he says, "it may have been regarded as a threat to some of the less affluent whites. Historically, there has been a correlation between tougher economic times and worsening race relations. Probably Montgomery County was never as liberal as it appeared to be."