The pathogenesis of renal interstitial fibrosis is not fully understood at present. As renal interstitial fibrosis occurs mainly in association with interstitial cell infiltrates, several cytokines, e.g. PDGF, TGF-beta, IL-6, are thought to play a major role in the induction and progression of renal interstitial fibrosis. In order to prove whether renal tubular epithelial cells produce cytokines which are known to modulate renal interstitial fibroblasts, RNA was isolated from human renal tubular epithelial cells and from human renal fibroblasts. The expression of specific cytokines and growth factors was analysed by Northern blot analysis using cDNA probes for IL-1 alpha, IL-1 beta, IL-6, INF-gamma, TNF-alpha, a-FGF, b-FGF, GM-CSF, PDGF-B and oligonucleotides for IL-2 and TGF-beta 1. In renal tubular epithelial cells from normal and diseased kidneys, mRNA transcripts for PDGF-B, IL-6 and GM-CSF could be detected. Quantitative analysis of the mRNA transcripts of the cytokines expressed revealed consistently higher amounts of GM-CSF and PDGF-B mRNA in tubular epithelial cells from diseased in comparison to normal kidneys. Furthermore, higher amounts of GM-CSF and PDGF-B mRNA transcripts were detected in tubular epithelial cells derived from kidneys with interstitial fibrosis as compared to tubular epithelial cells from diseased but nonfibrotic kidneys. Renal fibroblasts from normal and fibrotic kidneys showed a weak signal for IL-6, but PDGF-B and GM-CSF were not expressed in these cells. By means of colony assays, using human bone marrow cells in the presence or absence of neutralizing antibodies, the release of biologically active colony-stimulating factors GM-CSF and M-CSF by renal tubular epithelial cells from normal and diseased kidneys could be demonstrated. The present results probably indicate interactions between renal tubular epithelial cells and renal fibroblasts in the progression of renal diseases.