Glomerular cells grown in fibroblast growth factor (FGF) plus calf serum showed morphologic properties different from those of cells grown in calf serum (CS) alone. Cells grown in low concentrations of CS plus FGF grew as a monolayer in which the cells were closely apposed and nondividing, since they did not form the multiple layers seen in cells grown in high concentrations of CS alone. Cells shifted from growth in FGF plus CS to growth in CS alone showed morphologic features similar to those of differentiated glomerular epithelial cells, except that the cells were larger and more flattened. Indirect immunofluorescence studies using antibodies prepared against purified fibronectin or laminin demonstrated that subcultured secondary glomerular cells grown in CS alone had no detectable fibronectin or laminin on their cell surface, while cells grown in CS plus FGF showed these proteins at regions of cell-substrate contact. The differences between cells grown in CS alone and those grown in CS plus FGF were also demonstrated by the growth of subcultured glomerular cells on purified extracellular matrix proteins (fibronectin, laminin, or Type IV collagen) or purified heparan sulfate. Low-density subcultured glomerular cells did not grow well on plastic alone, even in the presence of fibroblast growth factor, but grew well on plastic coated with extracellular matrix molecules. Secondary cells grown in CS alone required laminin or Type IV collagen, whereas cells grown in CS plus FGF required fibronectin or heparan sulfate.