Peripheral blood lymphocyte subpopulations were studied sequentially during the gestation and the postpartum period of 84 obstetrically normal women. Both the percentage and absolute numbers of total peripheral blood lymphocytes decreased in the second and third trimester. Although there was a further decrease in percentage numbers 1 wk postpartum, the absolute numbers returned to normal. Absolute numbers of OKT3+ and of OKT4+ cells decreased in the second and third trimesters; OKT8+ cells decreased in the third trimester only. Absolute numbers of all T cells returned to normal postpartum. The absolute numbers of B lymphocytes, while showing no consistent pattern, decreased significantly in the third trimester and in the second postpartum period; however, the percentage values of all lymphocyte subpopulations did not alter during or after pregnancy. The data do not support the concept that a numerical imbalance in the T cell subpopulations has a causative role in the immunodeficiency of pregnancy.