Monoclonal antibodies to intact hCG and free beta-subunit of hCG permit the recognition of different individual antigenic sites on the hCG molecule. At least seven different epitopes may be recognized on the native molecule and a further two on the free beta-subunit. These antibodies were used in a mouse Leydig cell bioassay system to compare the degree of inhibition of hCG-induced testosterone production. Two antibodies were particularly potent at inhibiting hCG action, suggesting that they bind near to the receptor-recognition site on the hCG molecular. One antibody had little effect on biological action and was presumably binding distant from the biologically active site on the hCG. Combinations of monoclonal antibodies in immunoradiometric assays were used to develop highly sensitive and specific assays to intact hCG, free beta-subunit of hCG, and beta-subunit as part of intact hCG. Using these assays it was possible to detect 0.1 ng/ml hCG in the presence of high levels of LH. In 106 serum samples from pregnant woman free beta-subunit was considerably higher in samples with low concentrations of intact hCG, suggesting that free beta-subunit is not a limiting factor in placental production of intact hCG in early pregnancy. Comparison of urinary to serum ratios of hCG and free beta-subunit using specific immunoradiometric assays showed a good correlation for intact hCG but not for free beta subunit which was present in very high concentrations in urine.