Binding of radiolabeled human growth hormone, bovine prolactin, and ovine prolactin to membranes prepared from ovine and bovine mammary gland and liver was studied. Of these lactogenic hormones, human growth hormone exhibited the greatest total and specific binding capacity to either liver or mammary membranes. Characterization of binding assay conditions of human growth hormone indicated: that divalent ions (calcium or magnesium) were required for maximal binding, that binding was time dependent and saturable, that specific binding was proportional to the quantity of membrane protein assayed, and that bound radiolabeled human growth hormone was displaced similarly with either nonradiolabeled bovine prolactin or ovine prolactin. Interpretation of computer analysis of Scatchard plots derived from displacement curves indicated heterogeneous binding sites in liver and mammary membranes. Mean apparent dissociation constants of the high affinity binding sites ranged from 2.7 to 5.4 X 10(-9) M in mammary and liver membranes, respectively. Compared with mammary membranes from nonlactating ewes, specific binding of human growth hormone was increased 50% on day 100, 190% on day 130 of gestation, and 296% on day 60 of lactation. We conclude that radiolabeled human growth hormone can be used as a probe to measure lactogenic hormone binding sites in liver and mammary membranes from cows and sheep.