BACKGROUND Kidney mesangial cells are capable of producing and responding to interleukin 6 (IL-6) . In experimental glomerulonephritis mesangial cell proliferation correlates with increased IL-6 production. To investigate the involvement of IL-6 in post-nephrectomy compensatory hypertrophy, we studied the capacity of mesangial cells from single remaining kidneys to secrete IL-6 in culture. METHODS Mesangial cells were obtained from uni-nephrectomized or sham-nephrectomized Charles River rats. Cell cultures were maintained for 8 days in DMEM/FI2HAM medium supplemented with IL-1 of interferon (IFN). IL6 production was measured using an IL-6-dependent B9 human hybridoma cell line. RESULTS IL-6 production by mesangial cells from normal kidneys was significantly enhanced by IL-1, compared to unstimulated cells (p<0.01), and the increase was significantly greater in mesangial cells from a single remaining kidney (p<0.01). All cultures grown in control medium or with addition of IFN produced similar amounts of IL-6. CONCLUSIONS Mesangial cells from single remaining kidneys in culture maintain an exaggerated capacity to produce IL-6 in response to IL-1. IL-6 was reported to enhance or inhibit mesangial cell proliferation in vitro. We suggest that the local over production of IL-6 by a single remaining kidney may play a role in regulating a sequence of physiological events in compensatory renal growth, initially stimulating mesangial cell proliferation and later blunting the process.