Epidermal growth factors receptor (EGFR) was localized immunocytochemically in the testes of mature and immature rats and immature monkeys. One polyclonal antibody, recognizing the intracellular domain (RK2) of the receptor, was used to carry out the EGFR immunodetection. The RK2 antibody revealed the presence of the EGFR predominantly in Sertoli cells of mature and immature rats and of immature monkeys, although limited interstitial localization of the EGFR was also discerned in the mature rat. In cultured Sertoli cells of immature rats, grown in the absence of epidermal growth factor (EGF), the EGFR was randomly distributed at the cell surface, whereas after the addition of EGF the receptor became aggregated into distinct focal regions. In addition, EGFR of cultured Sertoli cells exhibited autophosphorylation activity upon stimulation with EGF, but failed to transcytose iodinated EGF across a permeability barrier formed by the cultured cells. Instead, all of the added iodinated EGF was internalized and degraded.