Platelet sensitivity to prostacyclin was investigated in platelet-rich plasma obtained from 12 pregnant women, 5 patients with cancer of the uterine cervix and 5 normal female controls. ADP-, collagen- and arachidonic acid-induced aggregation, as well as spontaneous platelet aggregation, were estimated. It was shown that platelets from pregnant women and, above all, from women with uterine cancer, are less sensitive to the inhibiting effect of prostacyclin on platelet aggregation. Thus, decreased sensitivity to prostacyclin seems to be another aspect of altered platelet function found in pregnancy and malignancy.