New murine monoclonal antibodies to a partially purified CA 125 antigen were developed and identified as M 2 and M 11. With immunohistochemical techniques, these new antibodies and OC 125 antibody were used to search for CA 125 in embryonic tissues and adult apocrine sweat glands and mammary glands. The embryonic skin, the periderm, expressed CA 125 antigen and its adult derivatives, the mammary glands and apocrine sweat glands, expressed CA 125 while in the active state of secretion. In a 6-week-old formalin-fixed and paraffin-embedded ectopic embryo specimen, antibodies M 2 and M 11 recognized CA 125 in the periderm, the notochord, the myocardium, the pericardium, the gastroenteric tract, enteric duct remnants in the umbilical cord (vitelline and allantoic ducts), the mesonephric duct, and the amnion. OC 125 staining of these formalin-fixed specimens was either very faint or absent. In a formalin-fixed and paraffin-embedded specimen of apocrine sweat glands from the axilla, antibodies M 2 and M 11 detected CA 125 antigen intracellularly in the secretory cells. Again no staining was observed with OC 125 antibody. In a frozen and acetone-fixed specimen of lactating mammary glands, antibodies M 2 and OC 125 detected CA 125 antigen intraductally. Colostrum and milk collected from 25 mothers at various stages post partum had mean CA 125 levels of 34,213 U/ml in colostrum, 1469 U/ml at 3 to 7 days, and 105 U/ml at 5 to 26 weeks.