The cancer antigen CA 125 is manifest by serous cystadenocarcinoma of the ovary and to a lesser extent by other gynecologic and nongynecologic tumors. Its presence was screened for in normal human fetal tissues and fluids. Appreciable quantities of CA 125 were discovered in amniotic fluid by both a dot blot assay and the commercially available immunoradiometric assay kit. The most likely source of this antigen was found not to be the developing fetus, since antigen was absent from cord blood and fetal urine, but rather the chorionic membrane, which contained significant quantities of the antigen. CA 125 was found in extracts of maternal decidua, but none was found in extracts of placenta or amnion. The CA 125 antigen was determined by gel filtration experiments to be in excess of 700,000 daltons and probably in the range of 2 to 3 X 10(6) daltons. Size heterogeneity based on gel filtration and anion heterogeneity based on anion exchange chromatography have both been demonstrated for the CA 125 molecule. The amniotic fluid antigen is composed of two subunits of approximately 240,000 and 180,000 daltons as detected by iodine 125-labeled OC 125 monoclonal antibody. The antigen may contain additional subunits not detected by the monoclonal antibody. Size and change heterogeneity as well as the poor definition of the subunit bands on polyacrylamide gels also suggest this molecule contains an appreciable carbohydrate component.