Corneal stromal fibroblasts expressed HLA-DP, -DQ and -DR Class II MHC antigens in response to interferon-gamma, but did not induce proliferative responses by allogeneic peripheral blood mononuclear cells in vitro when used as stimulator cells in a mixed leukocyte-type reaction. Furthermore, corneal stromal fibroblasts inhibited mixed leukocyte reactions between peripheral blood mononuclear cells of allogeneic donors, even when the corneal stromal fibroblasts were separated from the peripheral blood mononuclear cells by a 0.4 micron pore membrane. Pretreatment of the corneal stromal fibroblasts with interferon-gamma increased the inhibitory activity. Both [3H]thymidine incorporation and interleukin-2 production were inhibited, and the inhibition appeared to be mediated by a soluble factor whose production required protein synthesis. The inhibitory activity was not abolished by including 1-10 micrograms ml-1 indomethacin in the culture media. No inhibition was observed in the proliferation dose-response curves of responder peripheral blood mononuclear cells that had been cultured with corneal stromal fibroblasts for 3 days, prior to culture with allogeneic stimulator peripheral blood mononuclear cells. Thus, the ability of corneal stromal fibroblasts to interfere with alloimmune responses in vitro was dependent upon the continued presence of the fibroblasts and their continued production of a soluble inhibitory factor or factors. Inhibitors of allogeneic reactions that are produced by corneal stromal fibroblasts stimulated by immune cytokines (e.g. interferon-gamma) may play a role in prolonging corneal allograft survival.