Fibroblast cultures from normal human kidneys (NKF cells) and kidneys affected with interstitial fibrosis (FKIF cells) were analyzed for in vitro growth, differentiation dynamics, and collagen synthesis. FKIF cells are characterized by hyperproliferative growth, resulting in a prolonged mitotic lifespan, by an altered differentiation pattern, and by the expression of the FKIF cell-specific protein “fibrosin” (molecular weight 53 kd, isoelectric point [p_i] 6.1). Furthermore, FKIF cells synthesize four to five times more total collagen per cell as compared with NKF cells, and the relative amounts of the collagen types produced (type I, III, and V) are significantly different from controls. Thus, the in vitro cell system of FKIF cells may help to elucidate the underlying mechanisms triggering the induction and progression of renal interstitial fibrosis in vivo.