Well-characterized human glomerular mesangial cells were stimulated in vitro with interferon-gamma (IFN-gamma), interleukin-1, and tumor necrosis factor alpha (TNF-alpha) to determine the influence of these cytokines on the expression of class I and II HLA antigens. IFN-gamma induced a dose-dependent increase in expression of both class I and class II HLA antigens. TNF-alpha induced only class I antigens when used alone, but acted synergistically with IFN-gamma in the expression of class I and II antigens. Interleukin-1 had little effect on HLA antigen expression. These results raise the possibility that stimulated mesangial cells may act as antigen-presenting cells and may mediate local immune reactions in the glomerulus.