A ntibodies against the insulin receptor (Anti-R), which are found in the serum of type B patients with the syndrome of insulin resistance and acanthosis nigricans, inhibit the binding of insulin to its receptor and mimic the actions of insulin when studied acutely in vitro. After prolonged exposure of 3T3-L1 cells to Anti-R, the insulinomimetic activity is lost, and the cells show a marked decrease in their maximal response to insulin (antibody-induced desensitization), thus providing a model for the insulin resistance seen in vivo. This study explores in detail the mechanism and specificity of desensitization in 3T3-L1 cells.Desensitization, like the insulinomimetic activity of Anti-R, requires bivalence. Monovalent preparations of Anti-R inhibit insulin binding and shift the insulin biological dose-response curve to the right, but do not decrease the maximal insulin response. The affinity of monovalent Anti-R is less than that of the native antibody. Cross-linking of monovalent Anti-R reconstitutes its insulinomimetic activity and partially reconstitutes desensitization.Desensitized cells are resistant to the insulinomimetic actions of concanavalin A, which interacts with the insulin receptor, but are not desensitized to spermine and vitamin K₅, insulinomimetic agents that are thought to act independently of the insulin receptor. Glucose, pyruvate, or certain hexoses are required in the incubation media for desensitization to occur. Although Anti-R is taken up into cells and degraded by lysosomes, chloroquine, cycloheximide, colchicine, and cytochalasin E have little influence on the induction of or recovery from antibody-induced desensitization. These data suggest that desensitization is not merely due to the inhibition of insulin binding, but is a complex process involving a decreased ability of the receptor to generate a biological response.