Prepared as part of the Bureau of Labor Statistics' periodic reassessment of its projections of the future growth trends of the various sectors of the American economy, new labor force projections to 1990 are presented based on trends in labor force participation as observed through 1975 and on the most recent population projections of the U.S. Census Bureau. Major points of emphasis are the following: (1) The U.S. work force, which has been growing at unprecedented rates since World War II, is expected to grow more slowly during the next 15 years. (2) Projections developed by the Bureau of Labor Statistics (BLS) show that the labor force should grow at an annual rate of 1.9% in the latter half of the 1970's and only 1.1% a year during the 1980's, compared with a rate of increase of 2.3% during the first half of the 1970's. (3) Behind this slowdown is a sharp drop in the birth rate of the 1960's, which means there will be fewer youths reaching working age in the 1980's. The authors discuss specific 1975-1990 labor force participation projections of women, men, youth (16-24), prime age individuals (25-54), and older age persons (55 and over); compare present with earlier BLS projections published in 1973; explain methods of driving projections and underlying assumptions; and suggest alternative projections based on two different fertility assumptions. A supplementary table is appended which shows noninstitutional population, total labor force, and labor force participation rates by age and sex, actual for 1970 and 1975 and projected for 1980, 1985, and 1990. (EM)