Recently, highly purified bovine thyrotropin (bTSH) of pituitary origin, as well as recombinant human (h) TSH free of lutropin (LH) contamination, has been reported to activate the LH/choriogonadotropin receptor (LH/CGR). These data challenge the concept of TSH specificity for its own receptor. We, therefore, re-evaluated these data using, as targets, the recombinant hTSH and rat LH/CGRs stably expressed in Chinese hamster ovary (CHO) cells. Partially purified bTSH (2 IU/mg protein) and, to a lesser degree, highly purified bTSH (30 IU/mg protein) increased intracellular cAMP levels in CHO-LH/CGR cells (an EC50 of 0.2 and > 20 mIU/ml, respectively). In contrast, recombinant hTSH (up to 1 IU/ml) did not. All three TSH preparations increased cAMP levels to the same extent in CHO-TSHR cells (an EC50 of 0.3 mIU/ml). Furthermore, we observed only nonspecific, low affinity TSH binding for CHO-LH/CGR cells and also for CHO cells transfected with the expression vector alone (a Kd of 100 nM), although both high and low affinity TSH binding was demonstrated in CHOT-SHR cells (a Kd of 0.3 and 100 nM, respectively). These data indicate that even highly purified bTSH of pituitary origin contains significant amounts of LH, and that TSH itself does not appear to activate the LH/CGR.