Rat glomerular epithelial cells were cultured with human monocyte supernatant or with recombinant cytokines. A primary glomerular culture and a glomerular epithelial cell culture were made; supernatant from monocyte cultures derived from healthy humans, and recombinant tumour necrosis factor alpha (TNF alpha) or recombinant interleukin 1 beta (IL-1 beta) were added. Cell proliferation rates were assayed by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. In serum-free media, consistent proliferation of glomerular epithelial cells (GEC) was observed throughout the 3 week culture period. Significant growth-stimulatory effects were induced by lipopolysaccharide-treated monocyte conditioned medium and by 1-50 ng/ml of TNF alpha, growth being up to 400% more than in the control culture. The effect of TNF alpha depended mainly on its interaction with epidermal growth factor (EGF). In contrast to TNF alpha, IL-1 beta inhibited GEC proliferation; this was due to the early appearance and proliferation of mesangial cells, despite the culture being serum-free. This study showed that activated monocytes secrete growth factors for GEC in vitro, and that interaction between both TNF alpha and IL-1 beta and between TNF alpha and EGF can modulate GEC proliferation. These findings suggest that, under pathological conditions, monocytes or macrophages affect GEC proliferation, probably being involved in crescent formation.