The insulin receptor for human placental membranes has been solubilized in Triton X-100 and its properties have been examined in detail. Binding of [125 I]iodoinsulin to the soluble receptor is markedly inhibited by increas-ng concentrations of Triton X-100, due to a fall in receptor affinity. In 0.02--0.10% Triton X-100, the soluble receptor exhibits all the essential characteristics of the intact or particulate receptor. These include strict specificity for insulin and its analogues, increase in steady state binding with decrease in temperature, a pH optimum at 7.8--8.0, and negatively cooperative site-site interactions. The initial association rate of [125 I]iodoinsulin and the soluble receptor is a direct function of temperature, but the level of steady-state binding is lower at higher temperatures due to a marked increase in dissociation rate. Scatchard binding plots are curvilinear and show a large increase in affinity at 4 C with no change in total binding capacity (R0); increased binding to the particulate placental membrane at 4 C is due chiefly to an increase in R3. Negative cooperatively in the soluble receptor has been confirmed by kinetic experiments; thus, the dissociation of [125I]iodoinsulin from the receptor in the presence of "infinite" dilution is accelerated in the presence of 10(-8) M unlabeled insulin. The apparent molecular weight of the placental receptor, determined by gel filtration on 6% agarose, is approximately 300,000. These studies show that the basic properties of the insulin receptor do not depend on it being an integral conponent of the cell membrane.