The distribution of types I, III, and IV collagens, fibronectin, laminin, and heparan sulfate proteoglycan during human fetal kidney development has been studied by indirect immunofluorescence. Fibronectin and interstitial collagens types I and III are present in undifferentiated mesenchyme, whereas the intrinsic basement membrane components, type IV collagen, laminin, and heparan sulfate proteoglycan are not detected. In differentiated cortex, types I and III collagens are detected only as interstitial connective fibers, whereas fibronectin is found in both interstitium and glomeruli where its distribution is dependent on the stage of maturation. It is found in both the mesangium and along capillary walls of immature glomeruli, and principally in the mesangium of mature ones. Basement membrane components delineated the branching ureteric bud. They also outlined the structures of the earliest stage of nephrogenesis (epithelial differentiation). In mature nephrons, they are found along glomerular, capsular, and tubular basement membranes.