Plasma membranes derived from rat thyroid tumor (1-8R) which is unresponsive to thyrotropin but is responsive to dibutyryl adenosine 3':5'-cyclic monophosphate bind less than 20% of the [125I] thyrotropin which can be bound to plasma membranes from normal rat thyroids under conditions which optimize tumor membrane binding relative to normal thyroid membranes. In addition, the binding is different from thyrotropin binding to normal thyroid membranes both in its altered sensitivity to changes in hydrogen ion concentration and in a decreased sensitivity to competition by unlabeled thyrotropin. This reduced capacity to bind [125I] thyrotropin cannot be attributed to degradation of the hormone by membrane-associated proteases. Although the supernatant phase of the thyroid tumor homogenates contains a soluble component which inhibits [125I] thyrotropin binding to thyrotropin receptors on plasma membranes, its level is the same as in homogenates of normal thyroid tissue. Trypsin digestion does not expose thyrotropin receptors in a manner analogous to that seen in normal thyroid tissue. The major ganglioside in the tumor membranes is N-acetylneuraminylgalactosylglucosylceramide and the membranes lack the N-acetylgalactosaminyltransferase required for the synthesis of more complex gangliosides. In contrast, the normal rat thyroid membranes contain more complex gangliosides such as galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide and N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosyl ceramide as well as the glycosyltransferase activities required for their syntheses. Galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide can also be detected in normal membranes, but not in tumor membranes, by selective labeling with galactose oxidase (D-galactose: oxygen 6-oxidoreductase, EC 1.1.3.9) and [3H] sodium borohydride. These results support the hypothesis that gangliosides are important structural or functional components of thyrotropin receptors on thyroid plasma membranes.