The high concentrations of prolactin (hPRL) in human amniotic fluid appear to be derived principally from maternal decidua. The present study evaluated both the biologic and immunologic activity of amniotic fluid hPRL obtained from normal and selected complicated pregnancies. Biologic and immunologic activities of amniotic fluid hPRL were also compared with lecithin: sphingomyelin ratios and phosphatidylglycerol content. No significant correlation existed between amniotic fluid hPRL activities and fetal lung maturation. However, a significant increase in amniotic fluid hPRL concentration as well as specific biologic activity of the hormone was found in pregnancies complicated by hypertension. These findings suggest alterations in the synthesis of decidual hPRL and/or its transport to the amniotic fluid that may influence pregnancy-induced hypertension.