Macrophages may modulate mesangial expansion following renal injury via secretory products. We undertook the present study to determine the effects of macrophage supernatants on mesangial cell proliferation. Macrophage supernatants collected in serum-free media after 24 hours caused significantly enhanced mesangial cell proliferation in long-term culture at concentrations up to 50% but caused suppression at higher concentrations (control, 122,000 +/- 14,000 cells/well; 50% supernatant, 188,000 +/- 15,100 cells/well, P < 0.02 compared to control, n = 4; 80% supernatant, 52,000 +/- 3,500 cells/well, P < 0.01 compared to control, n = 4). In short-term culture [3H]thymidine incorporation, a measure of DNA synthesis, was significantly enhanced compared to control at supernatant concentrations up to 30% (30% supernatant, 4,120 +/- 310 cpm/well; control, 3,210 +/- 97 cpm/well, P < 0.5, n = 4), but uptake was reduced at high concentration (80% supernatant, 2,900 +/- 74 cpm/well; control, 3,210 +/- 97 cpm/well, P < 0.05, n = 4). When macrophage supernatants were collected after 48 hours incubation and incubated with mesangial cells, mesangial cell thymidine uptake was significantly suppressed compared to control (48-hour supernatant, 4,060 +/- 260 cpm/well; control, 5,890 +/- 270 cpm/well, P < 0.01, n = 4) and compared to 24-hour supernatants, which enhanced uptake (24-hour supernatant, 8,080 +/- 340 cpm/well; control, 5,890 +/- 270 cpm/well, P < 0.01, n = 4). Our results suggest that macrophage supernatants can directly enhance mesangial cell proliferation in vitro in both short-term and long-term culture, though this effect is lost at high concentrations of supernatant. These data lend support to the potential role of the macrophage in mediating mesangial expansion following renal injury.