Human growth hormone binding sites from female rabbit kidney microsomes were solubilized by treatment with the nonionic detergent Triton X-100. The binding of 125I-labelled human growth hormone to the solubilized sites retains many of the properties observed in the particulate fraction, such as saturability, reversibility, high affinity and structural specificity. The association and the dissociation process are time- and temperature-dependent. The association rate constant, k1, is 1.6 . 10(7) mol-1 . 1 . min-1 at 25 degrees C, and the dissociation rate constant, k-1, is 2.8 . 10(-4) min-1 at 25 degrees C. Solubilization causes an increase in affinity as well as in binding capacity. Scatchard plots from saturation curves suggest the presence of a single class of binding sites with a dissociation equilibrium constant, Kd, of 1.3 . 10(-11) M and a binding capacity of 133 fmol/mg of protein. Similar results were obtained from competition experiments. Specificity studies revealed the lactogenic characteristics of the solubilized sites. The Stokes radii of the free binding sites and of the 125I-labelled human growth hormone-binding site complex, determined on a Sepharose CL-6B column, are 57 and 53 A, respectively.