One hundred endometrial biopsies of various histological patterns, and material from 10 tubal pregnancies together with their associated uterine decidua, were examined for the presence of human placental lactogen using affinity-purified first and second antibodies and an indirect immunoperoxidase technique. Positive cells in endometrial curettings were seen only in association with an intrauterine pregnancy and morphologically resembled syncytiotrophoblast. Decidua associated with tubal pregnancy, pseudodecidua in progestogen-treated patients, and proliferative, secretory, and basal endometria were all negative. An immunoperoxidase stain for human placental lactogen is a useful marker for intrauterine pregnancy and supports the placental origin of the syncytial giant cells in so-called 'syncytial endometritis'. The technique is of potential value in those endometrial biopsies where pregnancy is suspected but no villi are seen.