To characterize the insulin receptor, membranes of cultured human lymphocytes (IM-9) were solubilized and subjected to quantitative polyacrylamide gel electrophoresis at pH 7.4 in 0.1% Triton X-100. A single major band with insulinbinding activity was found. The active protein by the criteria of gel electrophoresis exhibited a molecular mean geometric radius of 68 Å, corresponding to a globular protein (in Triton X-100) with an estimated molecular weight of 106 and a molecular net charge of 97 net protons/molecule. The binding activity of the receptor eluted from the gel retained the curvilinear Scatchard plot similar to that observed with the solubilized and membranebound receptors and could also be precipitated with antireceptor antibodies. The insulin-binding activity peak showed a 50-fold increase in specific binding activity when compared to the unfractionated solubilized membranes. Similar patterns of activity were observed with insulin receptors in rat liver, human placenta, and rat fat cell membranes. The insulin-degrading enzyme(s) of the human lymphocyte was also characterized by polyacrylamide gel electrophoresis. It has a radius of 38 Å, corresponding to a molecular weight of 200,000. The enzyme is fully distinct from the major peak of insulin-binding activity. A minor peak of binding activity, not exceeding 10% of the total, was associated with the degrading activity. These data demonstrate that the insulin receptor and the insulin-degrading enzyme(s) of the lymphocyte are distinct entities which may be characterized by quantitative polyacrylamide gel electrophoresis in Triton X-100 under nondenaturing conditions. The data also suggest the possibility for future isolation of the active insulin receptor by polyacrylamide gel electrophoresis in detergent-containing buffers. (Endocrinology106: 40, 1980)