Many observations suggest that cellular immunity is depressed during gestation. To define alterations in subsets of circulating lymphocytes during pregnancy, we compared the percentages of total T cells, T helper cells, and T suppressor cells in the peripheral blood of 31 pregnant women at various stages of gestation with those in that of nonpregnant controls. Control studies consisted of five women ingesting oral contraceptives and five women menstruating spontaneously. Isolated peripheral blood mononuclear cells were reacted with fluoresceinated monoclonal antibodies that were specific for Leu-2 cells, Leu-3 cells, and Leu-4 cells and that were subsequently analyzed on a fluorescence-activated cell sorter. No variation in the percentage of Leu-2, Leu-3, or Leu-4 cells was observed during the menstrual cycle or oral contraceptive cycle. No differences were seen when T-cell subsets at various stages of gestation were compared with the corresponding T-cell subset in nonpregnant controls. The data suggest that pregnancy does not alter the percentage of T-cell subsets in the peripheral blood.