Using the technique of somatic cell fusion, monoclonal antibodies to human chorionic gonadotropin (hCG) and its beta-subunit were produced. Spleen cells from immunized mice were fused with the myeloma line NS-1 using 50% polyethylene glycol and cultured in a selection medium. A sensitive enzyme immunoassay was performed for antibody screening using immobilized solid-phase antigen and anti-mouse IgG Fab' (rabbit)-beta-D-galactosidase complex. Three double-cloned hybridomas (named 4H10, 4G2 and 1D12) were obtained. Anti-hCG 4H10 antibody reacted with hCG, the alpha-subunit of hCG (alpha-hCG) and human luteinizing hormone (hLH); and anti-hCG 4G2 antibody reacted with hCG and the beta-subunit of hCG (beta-hCG). It was interesting that anti-beta-hCG 1D12 antibody had the capacity to bind with free beta-hCG but not with intact hCG, which suggests that the 1D12 antibody can recognize a determinant site that is unique to beta-hCG. This unique epitope in beta-hCG might be expressed in or near a part which is hidden in the intact hCG molecule which is composed of alpha- and beta-subunits.