The in vitro stimulation of peripheral blood mononuclear cells (PBMC) with interleukin 2 (IL-2) results in the development of potent cytotoxic effector cells, referred to as lymphokine-activated killer (LAK) cells. LAK cells are capable of lysing a wide variety of autologous, allogeneic and xenogeneic tumor cells. The exact mechanism of target cell recognition by LAK cells remains unknown. LAK cell activity has been reported for a variety of domesticated species except the horse. We report here that IL-2-stimulated equine PBMC, which fail to lyse either human or murine tumor cell lines, exhibit potent cytolytic activity against an equine tumor cell line, EqT8888. Cytolytic activity against the EqT8888 cells required 3 days of incubation with IL-2, was mediated primarily by T-cells, and was not restricted by major histocompatibility complex antigens. Though LAK activity could only be demonstrated using equine-derived target cells, xenogeneic targets could be lysed in a lectin-mediated cytotoxicity assay. The xenogeneic targets also failed to block LAK cell-killing of the EqT8888 cells in a cold-target competition assay. These results indicate that LAK cells in the horse appear to utilize a species-specific recognition mechanism during target cell lysis.