Platelets and leukocytes in venous blood were counted with automated methodology in 23 women longitudinally studied throughout pregnancy and at 6 wk after delivery. The average platelet count declined monotonically in a manner consistent with the gestational increase in blood volume. Total leukocyte count rose early in gestation and remained elevated, with neutrophils accounting for the changes. Absolute neutrophil counts during gestation were nearly twice those of the same women postpartum (5.76 vs. 3.13/mm3 .times. 10-3). The monocyte count also tended to increase, whereas lymphocytes, eosinophils and basophils declined in number.