In an effort to provide information on maternal immuno-compentence, a study was undertaken to measure lactin-induced transformation of maternal lymphocytes. Whole-blood culture was used. The phytohemagglutinin (PHA)-induced lymphocyte transformation of 137 pregnant women was significantly (P < 0.05 to 0.001) diminished until 32 weeks' gestation, and the concanavalin A (Con A)- induced lymphocyte transformation of 116 pregnant women was also significantly (P < 0.01 to 0.001) diminshed at all stages of gestation, as compared with those of 39 nonpregnant women. In addition, the Con A-induced lymphocyte transformation of ovarian venous blood and uterine venous blood from a pregnant woman was prominently diminished, as compared with that of basilic venous blood. It is suggested that there are maternal T-cell sub-populations that have different reactivities to PHA and Con A.