We have previously shown that highly purified urinary hCG has the potential to both stimulate the intracellular accumulation of cyclic AMP and induce growth of immortalized rat thyroid cells. We have now compared the ability of recombinant human TSH and purified urinary hCG preparations to stimulate Chinese hamster ovary (CHO) cells which have been transfected with the human TSH receptor. Only transfected CHO cells expressing recombinant TSH receptors, but not control CHO cells, were stimulated by hCG to release cyclic AMP in a dose-related manner and the effect of 100 IU of HCG was equivalent to approximately 9.2 uU of rec-hTSH. These data demonstrate that hCG interacts directly with the human TSH receptor.