To specify the role of mononuclear (MN) leukocytes in the development of mesangial cell proliferation, which is one of the main manifestations of glomerulonephritis, cell cultures of renal glomeruli of rats with nephrotoxic serum (NTS) nephritis were examined for the intensity of mesangial cell proliferation, the MN-leukocyte count, and for IL-1 production. The amount of mesangial cells and the intensity of their proliferation in the cultures of glomerular cells from rats with NTS nephritis were much higher than in intact rat cultures. At the same time the glomerular cultures from rats with NTS nephritis demonstrated an increase in the MN leukocyte count together with enhancement of IL-1 production. The treatment with prednisolone averted accumulation of MN leukocytes by the glomerular cultures and noticeably reduced mesangial cell proliferation. The supernatant liquid of cultures of peripheral blood MN leukocytes from patients with active nephritis suppressed human fibroblast proliferation, exerting no such action on mesangial cell culture. During the treatment with prednisolone, the supernatant liquid produced a reverse effect on fibroblast and mesangial cell cultures, which was associated with the clinical improvement of the health status. It is assumed that mesangial cell proliferation seen in nephritis may be related to infiltration of the glomeruli by MN leukocytes and to elevated production by them of IL-1 and that the therapeutic action of prednisolone may be determined by suppression of these processes.