Insulinlike growth factors (IGFs) are peptide hormones that regulate proliferation and differentiation of several types of normal and neoplastic cells. Recent studies of Wilms' tumor, a childhood kidney neoplasm, have detected increased expression of IGF-2 mRNA and protein. The present report describes detection of Type 1 IGF receptors in specimens of Wilms' tumor and adjacent nonneoplastic kidney tissue. These receptors recognize both IGF-1 and IGF-2, and binding of either peptide activates endogenous tyrosine kinase activity of the Type 1 IGF receptor beta subunit. These data indicate that Wilms' tumors contain receptors that recognize and respond to exogenous IGF in vitro, and that autocrine IGF production might contribute to the increased proliferation and abnormal differentiation of these cells in vivo.