Leukocyte counts and lymphocyte subpopulations were measured in umbilical cord blood obtained from normal pregnancies delivered either vaginally or by elective cesarean section. In the group delivered vaginally there were significantly higher median values for neutrophils, monocytes, and natural killer cells. The leukocytosis associated with labor is selective for mediators of nonspecific or innate immunity. Labor may be an immunologically beneficial process for the neonate.