To evaluated the usefulness of cultured human fibroblast for insulin receptor assay, the authors cultured fibroblast from biopsied normal adult female eyelid skin and assayed the insulin receptor with radioreceptor assay method. From the data obtained, percent of labeled insulin bound, numbers of insulin binding sites, affinity constants(Ka) and affinity of the empty sites(Ke) were calculated. The results were as follow; 1) The percent radioactivity bound of cultured fibroblast reached plateau at 4 hours $15^{circ}C$ incubation. 2) The scatchard plot of insulin binding to cultured human fibroblast was curvilinear and the affinity to receptor was decreased with increased receptor occupancy. 3) The numbers of high affinity, low affinity and total insulin receptor of cultured fibroblasts were 852, 24,800 and 25,652 sites per cell. 4) High and low affinity constants of cultured fibroblasts were $3.4 imes^{10}M^{-1},;and;1.08 imes10^8M^{-1}$, and the affinity of empty site was $5.0 imes10^8M^{-1}$.