Some properties of cytosolic receptors of the lactogenic type from female rat liver were studied and compared with those of membrane-bound (microsomal) receptors. The association constant between the cytosolic receptors and human growth hormone was 2.2 l/nmol, which was not significantly different from the value obtained for the microsomal receptors (3.6 l/nmol). Since unlabelled hGH and human prolactin, but not bovine growth hormone, displaced [125I]hGH bound to receptors from both sources, the cytosolic receptors, like the microsomal receptors, must be lactogenic. Furthermore, the cytosolic receptors were recognized by a monoclonal antibody raised against microsomal receptors from female rat liver. However, covalent cross-linking of cytosolic receptors to [125I]hGH and subsequent sodium dodecyl sulphate electrophoresis gave a single band corresponding to a molecular weight of 42 200 (after subtraction of the molecular weight of hGH), which differs significantly (p<0.01) from the values determined for the two distinct bands given by the microsomal fraction. Moreover, upon molecular sieve chromatography the receptor activity in the two fractions appeared at significantly (p<0.05) different elution volumes. These results show that the cytosolic and microsomal receptors have some structural features in common but are definitely not identical.