Glomerular cells grown in a defined medium consisting of Waymouth's MB 752/1 plus insulin, transferrin, sodium pyruvate, and nonessential amino acids were shown to require prostaglandin E1 and either fibronectin, laminin, or heparan sulfate to form a confluent monolayer. Under these conditions cells did not grow on Type IV collagen. Dibutyryladenosine-3,5-cyclic monophosphoric acid decreased the time needed for the cells to reach confluency. The results suggest a requirement for extracellular matrix molecules for growth of glomerular cells in defined medium, probably a requirement of adhesive molecules for cell attachment. The role of prostaglandins is less clear, but prostaglandin appears to be necessary for cell division. Since the cells growing in defined medium containing prostaglandin E1 and extracellular matrix molecules are epithelial in morphology, the authors suggest they may represent the stem cell precursors of either glomerular visceral or parietal epithelial cells, or both. The growth of glomerular cells in medium containing prostaglandins and extracellular matrix molecules is completely inhibited by sodium butyrate. Immunofluorescent studies show that the majority of cells show only faint staining for fibronectin along peripheral cell membranes; this result is in contrast to the bright stippled perinuclear pattern seen in cells grown in calf serum. The results are discussed in terms of the pathogenesis of glomerular lesions involving epithelial cells (e.g., crescents).