Chemical deglycosylation of human chorionic gonadotropin (hCG) produced an antagonist (DG-hCG) that specifically bound to hCG receptors but was no longer able to stimulate adenylate cyclase in the murine Leydig tumor cell line, MLTC-1. DG-hCG was restored to an agonist by incubating cells or membranes having the bound analogue with antibodies against hCG (anti-hCG). In the presence of anti-hCG, cyclic AMP accumulation and adenylate cyclase activity were stimulated over DG-hCG alone. There was no accumulation of cyclic AMP when the cells were exposed to anti-hCG alone or DG-hCG and normal serum or anti-hCG first then DG-hCG. Several different batches of anti-hCG were effective but their activity did not correlate with their affinity for DG-hCG or hCG. The effect of anti-hCG on DG-hCG activity was dose- and time-dependent. Maximal stimulation of cyclic AMP was achieved with antisera dilutions of 1:200 or less. When DG-hCG-treated cells were exposed to anti-hCG at 37 degrees C, there was a 10-min lag. The lag was eliminated when the cells were exposed to the antibodies at 4 degrees C for 3 h and then warmed to 37 degrees C. Adenylate cyclase was also activated when Fab fragments prepared by papain digestion of anti-hCG were used, whereas Fc fragments were not effective. Thus, the divalency of the anti-hCG is not the critical factor in the mechanism of antibody action. Our results suggest that anti-hCG converts DG-hCG from an antagonist to an agonist possibly by altering the conformation of the modified hormone.