Abstract The interaction of human growth hormone with human lymphocytes from an established culture (IM-9) was studied using 125I-human growth hormone. The binding of 125I-human growth hormone was rapid; with human growth hormone at 0.1 nm a steady state was observed in 90 min at 30°. Bound labeled human growth hormone was dissociated rapidly by addition of excess unlabeled human growth hormone. Binding of 125I-human growth hormone to cultured lymphocytes was relatively insensitive to alterations in the pH and in the concentrations of Ca2+, Mg2+, or EDTA. At 30° there was very little degradation of labeled human growth hormone or of the specific receptor sites. Tryptic digestion destroyed the capacity of cells to bind human growth hormone. The IM-9 cells bound all human growth hormone preparations but not unrelated hormones or nonprimate growth hormones. The binding of 125I-human growth hormone was inhibited 10 to 14% with 1 to 2 ng per ml of unlabeled human growth hormone and 50% with 30 to 40 ng per ml, well within the range of hormone concentrations in vivo. Analysis of steady state data revealed a single order of binding sites with an affinity constant of 1.3 x 109 m-1 and about 4000 binding sites per cell. Numerous human growth hormone preparations were assayed by use of this receptor system as well as by immunoassay and by bioassay in vivo. The potencies of the human growth hormone preparations determined by the radioreceptor assay correlated more closely with the bioassay than with the radioimmunoassay.