The present study was designed to compare the immunological, physical, and biological properties of native hCG with an hCG molecule secreted ectopically in vitro by an ovarian adenocarcinoma cell line maintained in long term tissue culture. The hCG produced by the cell line was concentrated by ultrafiltration of the tissue culture medium. The inhibition curves generated by serial dilutions of the culture medium concentrates were parallel to those obtained with purified urinary hCG in the beta-hCG RIA and the rat Leydig cell radioreceptor assay (RRA). The ectopic hCG also reacted with an antibody generated against the carboxyl-terminal peptide (109-145) of beta-hCG. The immunoreactive material cochromatographed with urinary hCG on a Sephadex G-100 column, as determined by the beta-hCG RIA and RRA. Neither free alpha nor free beta subunits were found in the tissue culture medium. The tissue culture gonadotropin was adsorbed onto a Concanavalin A-Sepharose column and could be eluted with alpha-D-methylglucoside. The biological activity of the ectopic hCG was 9289 IU/mg, as determined by the ventral prostate weight (VPW) method in hypophysectomized immature male rats. The biological to immunological ratios by the ventral prostate weight method and RRA were 1.79 and 2.17, respectively. The in vivo disappearance rate of ectopic hCG after injection into immature female rats was significantly faster than that of placental or urinary hCG, but was considerably slower than the disappearance rate of human LH. These studies demonstrate that the immunoreactive and biologically active portions of the hCG produced by the ovarian adenocarcinoma cell line and native hCG are similar or identical. The faster disappearance rate of the ectopic hCG in the rat model may be due to incomplete sialylation of the oligosaccharide moiety of the hCG molecule.