The metanephric mesenchyme is considered a homogeneous population of predetermined, butpluripotentcellswith a nephrogenic bias. By an inductivestimulus, the mesenchyn'1e is programmed to differentiate into the various epithelial phenotypes of the secretory nephron. IA fraction of the mesenchymal cells, however, remains inthe interstitium between the nephrons a~ d differentiates into spindle-shaped, clear-cytoplasmic renal stroma. We have analyzed the molecular nature of these cells in order to discover the specific cell types that could be involved in the morphogenetic processes during kidney differentiation. In situ hybridization reveals neurofilameht light protein mRNA, and immunohistology shows neurofilament light and medium proteins in the stromal cells around kidney tubules. Byimmunohistochemistry these peritubular stromal cells can be distinguished from the neuronal cells of the renal microganglion: the peritubular stromal cells a~ e neurofilament-positive but l1 neural cell adhesion protein-negative, whereas the neuronal cells with axonal extension are both neurofilament-positive and l1 neural cell adhesion protein-positiVe. Proliferation index of the stromal cells was low as compared to tubular cells, as shown by bromodeoxyuridine incorporation.