Recently, it has been shown that inflammatory and noninflammatory glomerular diseases are often complicated by changes in the tubulointerstitial system, mostly caused by renal interstitial fibrosis (IF). To analyze pathomechanisms of these tubulointerstitial lesions, growth behavior and expression of cell surface molecules of tubular epithelial cells and renal fibroblasts in culture, established using renal biopsies of IF, were studied. Furthermore, the capacity of renal fibroblasts in culture to produce extracellular matrix was investigated. Expression of MHC-class II antigens on proximal tubular epithelial cells (PTECs) has been induced by supplementing the culture medium with interferon gamma. Renal fibroblasts, the major cell types growing out from renal biopsies with interstitial fibrosis, form so-called hillocks in culture and synthesize collagen type I, III, and V. Further analysis using cocultures of tubular epithelial or renal endothelial cells on the one hand, and defined renal fibroblasts on the other, will clarify the interaction between these cell types, which probably will lead to a better understanding of the pathomechanisms causing disease progression.