Expression of insulin-like growth factor binding protein-1 (IGFBP-1) messenger RNA (mRNA) was studied in tissues of human fetuses from 15 to 23 weeks of gestation. Northern blot analysis revealed IGFBP-1 mRNA in the fetal liver and kidney but not in other fetal tissues, including the brain, heart, lung, skeletal muscle and spleen. Studies by in situ hybridization histochemistry showed that, in all fetal kidneys tested, the IGFBP-1 mRNA was localized preferentially to the epithelial cells of the collecting ducts, as well as to the cells of developing glomeruli and in the subcapsular nephrogenic mesenchyme. Less intense labeling for IGFBP-1 mRNA was seen in the connective tissue stroma of the medullary pyramids. A weak signal was detected in the mature glomeruli, and in the cells of the medullary mesenchyme and capsular connective tissue. IGFBP-1 protein was detected by immunoperoxidase staining mostly around small blood vessels but not in the respective endothelium. The protein was also present in many epithelial cells of the collecting ducts and in stromal connective tissue. These results show that the predominant sites of IGFBP-1 transcription in the developing kidney are those with most active differentiation.