Black and white workers in the United States are beginning to earn similar salaries because they are beginning to receive similar opportunities, especially in education, according to a study released yesterday.

Black men now earn three-fourths of the income of their white working counterparts, and the earnings of black and white working women are nearly equal, the study said.

Thirty years ago, black working men earned half the wages of white working men, and black women earned only one-third of the income of white women, according to the study by The Rand Corp. of Santa Monica, Calif., a nonprofit research institution specializing in national securtiy problems and domestic affairs.

The study deals only with blacks who are employed and does not address the issue of unemployment, which affects blacks disproportionately.

The authors of the study, James P. Smith and Finis Welch, argue that the improved economic status of black workers, especially that of black men, has come about "because blacks and whites [have] become increasingly alike" in "income-producing characteristics," such as education.

For example, in 1930, the "typical black male" entered the labor market with 3.7 fewer years of formal schooling than his white counterpart. By 1970, the black entry-level worker had 1.2 fewer years of formal education than his white competitor, the study said.

The study said black women had 2.6 fewer years of formal schooling than white women who went to work in 1930. But in 1970, black working women virtually equaled white working women in the number of years of schooling completed.

Smith and Welch said the rising levels of educational attainment accounted for 47 percent of the salary increase for black working men, and 33 percent for black women.

Affirmative action programs that bar job discrimination on the basis of race or sex have further improved employment and income opportunities for black women "because hiring a black woman met both race and sex quotas set for employers," the authors said. They added: "The affirmative action thrust may have given black women an advantage in the job market."

The authors said there is little evidence that the affirmative action employment programs have improved earnings for black men.

Also, according to Smith and Welch, in spite of the overall improvement in earnings for black men, it will be at least 30 years before their salaries equal those of their white counterparts. The authors argue that the "weight of the past" seems to bear more heavily on black men and will continue to slow their economic progress.

For example, they said that by 1835, every Southern state had a law forbidding black education, and that emancipation came at a time"when no slave under 30 years old could legally have been schooled."

"In 1960, the average black working male was 47 years old and was born in 1912. Assuming a 25-year generation span, his father was born in 1887 and his granfather in 1862. The perilstent effects of past education level affect the market performance of present generations through family background. Even as current education levels by race converge, the weight of the past will be a factor depressing relative wages" of black men, the authors said.

On another question, Smith and Welch said they found little to support the contentions of some economists that blacks, as a group, are funneled into dead-end jobs. Instead, they said their findings indicate that blacks and whites with comparable educational levels enter similar occupations and receive similar wage increases throughout their careers.

Black income is even beginning to fare better during economic downturns, which traditionally have affected black workers worse than whites, the authors said.