ABSTRACT The cystine-aminopeptidase (CAP)1) activity of placental origin (P-CAP) and of tissue origin (T-CAP) has been investigated in plasma from non-pregnant women, during normal pregnancy, in pregnancies complicated with severe pre-eclampsia and in plasma from the umbilical cord. In plasma from non-pregnant women the enzyme activity was significantly higher at pH 5.8 (T-CAP) than at pH 7.65 (P-CAP). During the pregnancy a gradual increase in both T-CAP and P-CAP took place. However, from about day 220 in the pregnancy the P-CAP became higher than the T-CAP. Between normal pregnancy and pregnancies complicated with severe pre-eclampsia there was only a low significant difference in the T-CAP activity, but highly significant difference in the P-CAP activity. The normal P-CAP pattern showed a gradual increase from about the 100th to the 270th day. A great spread in normal values was seen especially from about day 238 of gestation, due to variations in the enzyme activity of the women. The coefficient of correlation between the P-CAP and the crude placental weight was calculated to + 0.67. The half-life of P-CAP was calculated to 8.3 days. In plasma from the umbilical cord the T-CAP activity was significantly higher than the P-CAP, and was simultan that in plasma from non-pregnant women.