We examined whether injection of heterologous hormones for more than one week might evoke a humoral immune response which would simulate receptor induction. Male rats were injected daily for ten days with human growth hormone (hGH) or ovine prolactin (oPRL), and binding of 125I-hGH and 125I-oPRL was examined in serum and in membranes from liver and lung. Specific binding of 125I-hGH and 125I-oPRL increased in the sera of hGH- and oPRL-injected animals, respectively. A marked increase in hGH but not oPRL binding also occurred in crude membrane-preparations of tissues from hGH-injected rats. Similarly oPRL but not hGH binding increased in tissues of PRL-injected animals. Furthermore, binding activity solubilized from liver membranes of hormone-injected rats was precipitated with Staphylococcus aureus (protein A) indicating that the induced binding sites were immunoglobulin-like. Hence apparent up-regulation of lactogenic receptors following long-term treatment with heterologous hormones may be due to generation of anti-hormone antibodies.