In order to evaluate whether in vivo immune-mediated lesions in the glomerulus might affect growth of glomerular cells in vitro, we developed a combined in vivo-in vitro approach. An immune-mediated mesangial cell injury was induced in rats by intravenous injection of a rabbit anti-rat thymocyte antiserum (ATS). Beginning at day 1 up to 12 days after ATS, glomeruli were isolated for glomerular cell culture and evaluation of cell growth. Compared with control rats cell growth from nephritic glomeruli was significantly higher at all time points studied. Immunohistologic studies performed revealed that the proliferating cells had characteristics of glomerular mesangial cells. These data demonstrate that mesangial cells derived from nephritic glomeruli have increased growth in vitro compared with cells from normal glomeruli.