The interaction of 125I-labeled human growth hormone (HGH) with microsomal membranes of female rat liver is a specific, saturable and reversible process; at saturation, about 0.9 picomoles of 125I-HGH are bound per mg of membrane proteins. Equilibrium is achieved after 90 minutes of incubation. The binding is dependent on membrane concentration and pH of the incubation medium. Membrane treatment by phospholipase A causes a 1.5 fold increase in binding capacity. 125I-HGH exposed to membranes or eluted from the membrane-receptor complex retains biological activity as tested by binding to liver membranes. The variation in binding with the age of the female rat is probably related to a change in the binding capacity and not in the binding affinity. Microsomal membranes can be solubilized by the use of Triton X-100; solubilized membranes retain their capacity to bind HGH and the properties of the interaction are similar to the ones of the particulate membranes. Lactogenic and somatogenic receptors are eluted from an acrylamide-agarose column in the same position; their apparent molecular weight is 350,000.