Microsomal membranes from A875 human melanoma cells contain nerve growth factor receptors (NGF-receptors) which appear to belong to a single class with homogeneous binding properties, as determined by Scatchard plots. NGF-receptors in these membrane preparations are also uniformly highly sensitive to tryptic proteolysis, and “‘I-NGF bound to NGF-receptor in these membranes is rapidly dissociated in the presence of a high concentration of unlabeled NGF. However, analysis of “‘I-NGF dissociation kinetics indicated that two classes of NGF-receptor were present in these membranes. Thus, NGF-receptors can express either high or low affinity trypsinsensitive states in addition to the high affinity trypsin resistant NGF-receptor state described previously (Buxser, S. E., Kelleher, D. J., Watson, L., Puma, P., and Johnson, G. L. (1983) J. Biol. Chem. 258,37413749). The high affinity trypsin-sensitive and low affinity trypsin-sensitive states correlate with 200and 90-kDa ‘“I-NGF . NGF-receptor complexes observed in photoaffinity cross-linking experiments. The absence of differences in peptide maps generated from the two sizes of NGF-receptor proteins together with structural and binding data strongly indicates that the 200-kDa NGF-receptor protein is a complex, probably a dimer, consisting of two 80-kDa NGF-receptor proteins associated with a single &NGF dimeric molecule. A model is proposed which relates structural states of NGF-receptors with specific receptor binding properties. The model provides an alternative explanation for binding phenomena previously attributed to negative cooperativity.