1. GM1 was modified by removal of its fatty acid. The modified ganglioside was readily taken up by GM1-deficient cells as measured by increased cholera toxin binding or by uptake of a tritium-labeled derivative. Cells treated with this derivative accumulated more cyclic AMP when exposed to toxin than did GM1-treated cells even though equivalent amounts of toxin were bound to the cells. In addition, there was more activation of adenylate cyclase. Thus, the sphingosine moiety of GM1 appears to modulate toxin action. 3. Rat testes membranes were incubated with iodinated human chorionic gonadotropin (hCG) and then treated with cross-linking reagents. Covalently coupled hCG-membrane complexes were detected by inability of high salt or acid to displace the hormone. The detergent-solubilized cross-linked complex behaved on gell filtration column and sucrose density gradients similar to the uncross-linked hormone-receptor complex. The SDS-polyacrylamide gel electrophoresis, the crosslinked complex had a similar molecular size than by the above methods, thus, indicating that the receptor may be a multimeric complex.