Background/Aim: RF/J mice are a model of spontaneous immune complex mediated glomerulonephritis showing massive extracellular matrix accumulation and progressive glomerulosclerosis. The aim of this study was to investigate whether there is an altered cultured mesangial cell (MC) phenotype in RF/J mice associated with these glomerular changes. Methods: The nature of cultured MCs from RF/J mice in the proliferative response to platelet-derived growth factor (PDGF) BB was compared with that of normal mice (BALB/c) by <sup>3</sup>H-thymidine incorporation. The binding of PDGF-BB was examined with Scatchard analysis, and the messenger RNAs (mRNAs) of PDGF β-receptor, collagen I, collagen IV, and fibronectin were detected using Northern blot analysis in the MCs of each mouse. Results: The <sup>3</sup>H-thymidine incorporation of MCs from RF/J mice showed significantly lower responses to PDGF-BB stimulations with concentrations ranging from 0.5 to 10.0 ng/ml in comparison with those of BALB/c mice which exhibited a proportional dose- dependent increase of the incorporation (p < 0.05 for 0.5 ng/ml PDGF-BB, p < 0.01 for 1.0–10.0 ng/ml). According to the Scatchard analysis, MCs from BALB/c mice showed aKD of 105 p M of PDGF-BB binding to its receptors, and the density of receptors was 5.82 fmol/10<sup>5</sup> cells. However, no binding PDGF-BB site on the surface of MCs from RF/J mice was noted. Northern blot analysis of MCs from RF/J mice indicated negative expression of detectable PDGF-β receptor mRNA. As for matrix protein messages, MCs from RF/J mice did not express mRNA of type I collagen, but did express a higher amount of type IV collagen and fibronectin in comparison with MCs from normal BALB/c mice. Conclusions: An altered phenotype in MCs of RF/J mice was demonstrated, possibly contributing to the characteristic pathological glomerular changes. However, the precise association remains to be clarified.