In situ hybridization was used to localize the cells that produce erythropoietin (EP) in anemic murine kidneys. Kidneys from anemic and nonanemic mice were fixed and processed for paraffin embedding. Sections were hybridized with a 35S-labeled RNA probe complementary to mRNA coding for EP. An uncommon, but specific type of cell was intensely labeled in the cortices of anemic kidneys. The labeled cells were clearly nonglomerular and nontubular. Their location outside of the tubular basement membrane was consistent with that of a subset of interstitial cells or capillary endothelial cells.