Peripheral blood mononuclear cells from foals with hereditary severe combined immunodeficiency (SCID) have morphologic characteristics of large granular lymphocytes (LGL). Attempts to demonstrate cytotoxic activity were without success unless the LGL were incubated with 100 U of human recombinant interleukin 2 (rIL 2)/ml for 24 hr. With rIL 2 incubation, low effector to target ratios (10:1) consistently yielded high levels of cytotoxic activity (30 to 50%) in a standard 4-hr 51Cr-release assay using YAC-1 lymphoma or K562 erythroleukemia cell lines as targets. Monoclonal antibody EqT12 reacted with a large percentage of these cells, and the level of cytotoxic activity was directly correlated to the percentage of EqT12+ cells in the preparation. In normal horses, the percentage of circulating EqT12+ cells is low (5%), while at the same time, cytotoxic activity is not usually detectable even with rIL 2 incubation. In contrast, the percentage of EqT12+ cells in blood of SCID horses is high (up to 40%), as is the IL 2-inducible cytotoxic activity. These results indicate that cytotoxic cells with morphologic and functional characteristics of natural killer cells are produced by horses with SCID.