Specific studies of labelled insulin binding to insulin receptors on human placental crude membrane particulate were carried out. Specific binding of (125)I-insulin to human placental membrane particulate was affected by temperature and pH of the incubation buffer and by the protein concentrations of the insulin receptor. Scatchard analysis showed that human placental insulin receptor has two affinity constants, high affinity-low capacity component (K(1)) and low affinity-high capacity component (K(2)). The K(1) and K(2) values were 1.410×10(9) M(-1) and 0.257×10(9) M(-1), respectively. The average affinity (Ke) was 0.259×10(9) M(-1). Dissociation of labelled insulin from the placental insulin receptor demonstrated negative cooperativity. Specific binding of (125)I-insulin to the placental insulin receptor was not affected by the sera of patients with various auto-immune diseases, such as systemic lupus erythematosus, rheumatic arthritis, hyperthyroidism, and mixed connective tissue disease, or by the sera of patients with lipoatrophic diabetes and insulin dependant unstable diabetes. Furthermore the binding of labelled insulin to the placental insulin receptor was not related to various hormones such as glucagon, C-peptide, GH, ACTH, cortisol, GABA, L-DOPA, TRH, TSH, and testosterone. Anti-insulin receptor antibodies in a patient with insulin resistant diabetes were measured by RRA using placental membrane particulate and by PEG and immunoprecipitation using solubilized placental insulin receptor. The inhibition rate by polyethylene glycol method using solubilized insulin receptor corresponded well with the inhibition rate by RRA. Immunoprecipitation using solubilized insulin receptor and goat anti-human IgG is thought to be indispensable to detection of antireceptor antibodies in type B insulin resistant diabetes. The change in clinical features of the patient with type B insulin resistant diabetes correlated closely with change of titers of anti-receptor antibodies. Immunosuppressive therapy consisting of prednisolone and cyclophosphamide resulted in complete remission of clinical diabetes and disappearance of anti-receptor antibodies. Scatchard analysis showed that antireceptor antibodies decreased the affinity constant of the insulin receptor, but did not affect the numbers of receptors. This finding suggests that anti-receptor antibodies bind to near the insulin receptor and inhibit binding of insulin to the insulin receptor, thereby causing an extremely insulin-resistant condition.