Radioimmunoassays utilizing antisera specific for the carboxyl-terminal portion of human chorionic gonadotrophin (hCG) beta-subunit were used to measure the concentration in human pituitary extracts of an immunoactive hCG factor (hCG') which was different from human LH (hLH). The content of hCG' from different human pituitary pools collected between 1966 and 1979 was relatively constant at 0.5-1.1 microgram per gland. The hCG' concentrations observed in acetone-dried powder of individual human pituitary glands (0.4-26 ng/mg) were close to those reported for full-term placental powder. After separation and partial purification of human pituitary glycoprotein hormones, pituitary hCG' was found mainly in the crude human FSH (hFSH) fraction. It was separated from hFSH by diethylaminoethyl-cellulose chromatography at pH 7 and by gel filtration on Sephadex G-100. On gel filtration its molecular size was larger than that of hLH or hFSH and it was strongly bound to Concanavalin A-Sepharose. The most active preparations of pituitary hCG' obtained by these procedures were approximately 5 per cent as potent by specific radioimmunoassay as hCG purified from pregnancy urine. Although the hCG' content in individual pituitary glands was more variable than the hLH content, on average pituitary hCG' was estimated to be around 25- to 50-fold less than the content of hLH, hFSH or human TSH in the human pituitary gland.