Keratin-positive fibroblast-like epithelial cells (FLE), isolated from human corneo-scleral-conjunctival rims, were shown to inhibit mitogen-driven (concanavalin A) DNA synthesis by murine thymocytes and splenocytes [lymphoreticular cells (LRC)]. The effect exerted by live cells culture and by their supernatants was caused by factors active across species barriers. Paraformaldehyde-fixed or irradiated cells also suppressed mitogen-induced thymocyte DNA synthesis, but their supernatants manifested no such activity. Interaction between FLE cells and LRC the presence of the mitogen resulted in suppressed cellular activation as evidenced by significantly lowered tetrazolium salt (MTT) reduction in murine thymocytes and splenocytes, suggesting reduced mitochondrial activity. The suppressive effect was seen with live and paraformaldehyde- fixed FLE cells. There was a good correlation between MTT assays and [3H]thymidine uptake experiments. Suppression of MTT reduction in murine thymocytes and splenocytes by intact FLE cells could be reversed by the addition of interleukin-1 (IL-1). Indomethacin prevented FLE-conditioned medium-induced suppression but failed to relieve suppression by whole FLE cells. Thus, suppression of LRC function by FLE cells and their secretions appeared to operate by different mechanisms. One mechanism related to prostaglandins present in FLE cell-conditioned medium, whereas another mechanism appeared to involve cell-membraneassociated factor(s). The findings not only provide additional information on the capability of corneal cells regulate lymphoreticular cells but suggest an important role for IL-1 in the regulation of LRC function and corneal inflammation and immunity.