HE WAS THE ONE, who was supposed to be different - the bulldog federal prosecutor who would never cross the street to become a defense attorney the way so many others have. Now Barnet D. Skolnik, the assistant U.S. attorney in Baltimore who changed the face of Maryland politics, has switched.

Next month Skolnik - who over the past six years had led the prosecution of, among others, Vice president Spiro T. Agnew and Gov. Marvin Mandel - will begin sharing offices with Washington attorneys William G. Hundley and Plato Capheris, other former crack prosecutors who now specialize in defending politicians and businessmen accused of political and commercial corruption.

A big bear of a man who worked 100 hours a week on his cases, Skolnik developed the reputation of a zealot determined to root out corruption. His frequent moralizing during trials aroused in uncommon personal hatred among defendents, who felt he carried a special chip on his broad shoulder against politicans and their unsuccessful business friends.

It was this one-dimensional view of Skolnik, coupled with statements of his own, that led to the view that he would be a prosecutor forever.

After the Mandel conviction 13 months ago, for instance. Skolnik talked openly about moving on to new things. But the new things he talked of - at least to reporters - were always on the prosecutor's side.

"A lot of people, including reporters, have misread me for a long time," said Skolnik. "I enjoyed my work as a prosecutor, but I am a lawyer. I am not just a prosecutor. I was doing work as a prosecutor because it was the job I had. It is a misreading to say that I want to put everyone in jail or that I want to get all the politicians."

Nonetheless, Skolnik acknowledges he has changed.

"Three to five years ago I was a different person," he said. "I don't want to say that the guy I was then couldn't be a defense attorney, but there was more justification for saying that then than there is for saying it now. You get tempered over time."

Not only that, but you run up some bills over time, too, and assistant U.S. attorneys are not the most highly paid of lawyers - no matte how dedicated or how skilled they are.

"Barney has six kid," said Hundley, whose fees on the defense side runn higher than Skolnik's salary in the same case. "While the job is great, he reached the point of economic necessity where he had to move on," said Hundley.

Skolnik is sharing office spare, not becoming a partner of Hundley and Cacheris - partially to make sure there are no conflict-of-interest problems and partially to ee if the chemistry is right between the three men.

That means, for example, that Hundley and Cacheris will be able to take cases that originated in the Baltimore U.S. attorney's office while Skolnik still worked there. It also erases conflict of Mandel and his businessmen friends, which saw Hundley and Skolnik on opposite sides of the courtroom.

It also means that the established practice of Hundley and Cacheris will be available to give Skolnik a boost with client referrals. Lawyers can only represent one defendant in a corruption case, and since most of them involve large numbers of people - either as defendants or potential witness - the specialists in this booming field often are in the position of throwing clients to one another.

What will clients think of being defeated by a man they once regarded as an arch enemy?

"The best prosecuters make the best defense attorneys," said Hundley, who should know. For 14 years, he was one of the toughest U.S. prosecutors in the country - the head of Attorney General Robert Kennedy's organized crime section until he switched over in 1966. Now he is one of the most sucessful and sought-after defense attorneys.

"I used to have the same reputation as Barney myself" said Hundley, "but it's been so long ago people have forgotten.

Sydney Kentridge, the leading white legal defender of South Africans accused the crimes relating to that country's strict apartheid laws, will speak in Washington on Tuesday under the sponsorship of the Lawyer's Committee for Civil Rights' Southern African Project.

Kentridge, a native of Johannesburg, represented the family of South African black leader Steve Biko in the inquest into the cause of his death. Kentridge became the leading actor in the case, constantly brining out the details of how Biko was mistreated before he died while in police custody of head injuries a year ago.

While the political trials have won him the admiration of South African blacks and the hatred of that country's police, lawyers there say he really makes his money the way most advocates do - from lucrative civil suits and patent cases.

In the Biko inquest, Kentridge had the help of the Laywers' Committees' Southern African Project, which contributed both financial aide and technical support, including the services of Washington lawyers Peter J. Connell and Debevoise and Liberman and Federal Communications Commissioner Tyrone Brown.

Tuesday's luncheon will be held at noon at the International Club.

Short takes: William G. Malone, assistant general counsel of the Veterans Administration, has been elected president of the Federal Bar Association . . . Stefan A. Riesenfeld, counsel on international law to the legal advisor of the State Department and professor of law at the University of California Law School, Berkeley, will speak to the Washington Foreign Law Society Tuesday . . . Michael F. Colley, president of the Association of Trial Lawyers of America, will address a seminar for members of the 4th and D.C. Judicial Circuits on Thursday through Saturday at the Washington Hilton Hotel.