Most American workers earned $15 more a week in May than they did a year ago, but couldn't buy more for their money because of inflation, the Labor Department reported yesterday.

The department said weekly earnings of full-time wage and salary workers averaged $212 last May 8, an increase of $15 or 6.9 per cent over the previous 12 months.

"But because consumer prices rose at about the same pace - 6.7 percent - real earnings (purchasing power) did not change significantly," he department said.

The figures were based on an annual survey of 55,000 households taken by the Census Bureau each May for the Bureau of Labor Statistics. The survey showed sharp differences remain between earnings for men and women and between whites and black.

According to the Labor Deparment.

The median weekly earning of all men who work full time was $253 in May 1977, while the median for women in full-time jobs was $156 or about 62 percent of that for men. The ratio has shown little change over the past 18 years, despite the heavy influx of women into the work force.

The median is the level at which half the workers earned more and half earned less.

Married men working full time were earning an average of $272 a week in 1977. Married women with full-time jobs reported a median of $158.

Average weekly earnings of white full-time workers was $217, compared with $171 for Ibacks.

The median for white men, at $259, was 29 per cent higher than that of black men, whose median wage was $201. However, the $157 median wage form white women was only 7 percent higher than those of black women, whose median wage was $147.

The Labor Department said these race differentials were considerably larger 19 years ago - 44 percent for men and 5 percent for women.

Although whites continue to earn more than blocks the survey showed that real earnings or purchasing power, increased at a sharper rate for blacks over the the past 19 years than for white workers.