ABSTRACT: The total number of circulating leucocytes in the peripheral blood of pregnant women progressively increased with advance in gestation because of neutrophilia. When the phagocytic activity, representing nonspecific immunity, was estimated by a luminol-dependent chemiluminescence (CL) response during phagocytosing opsonized zymosan, we observed that the CL response of whole blood and Ficoll-Paque separated polymorphonuclear leucocytes (PMNL) significantly increased throughout the pregnancy (P<0.01). The CL responses of mononuclear leucocytes (MN) and monocytes also increased and reached peak levels in the third trimester (P<0.05). These findings suggest that the phagocytic activity in pregnant women increases, not only with regard to the number of phagocytes but also with regard to individual cell function, from a relatively early stage of the pregnancy, and that this increased nonspecific immunity may compensate in part for a weakened specific immunity of the maternal host. Attention should be directed to effects of human chorionic gonadotropin (hCG) relative to the increased CL response during pregnancy.