Radioreceptor assays (RRAs) provide useful information about the bioactivity of peptide hormones. We have developed a RRA for human GH using membranes prepared from human liver. The assay has 1% cross-reactivity with human PRL, 0.11% cross-reactivity with human chorionic somatomammotropin, and negligible cross-reactivity with bovine and rat GH. The assay has distinct advantages over the IM-9 lymphocyte RRA in having reduced nonspecific interference from serum proteins and providing a uniform source of receptors which can be stored for long periods at -70 C. The coefficient of variation of the assay is 13%. In studies of 55 sera from short children with normal or elevated serum GH concentrations the mean ratio of RRA GH to immunoradiometric assayed GH was 1.28. This assay provided important information about the total GH activity in pregnancy. There was a progressive rise in RRA GH during pregnancy, reaching 64.7 +/- 2.5 (+/- SE) micrograms/L at term. This level is 2-3 times higher than suggested by earlier estimates made with monoclonal antibody RIAs for pituitary GH. At term the contributions to total GH activity of serum are less than 3% from the pituitary, 12% from human chorionic somatomammotropin reacting with the GH receptor, and 85% from placental GH. Despite this great increase in receptor-reactive GH, somatomedin activity by bioassay is reduced, and IGF-I, determined by RIA after acid-gel filtration, is minimally elevated.