Two murine monoclonal antibodies reactive with porcine nylon wool-nonadherent T cells were examined by flow microfluorometry analysis for cellular distribution, and by functional analysis for their effect on MLR and CTL assays. The two determinants recognized, called PT4 and PT8, were shown to be expressed on almost mutually exclusive cell populations, with only 6 to 15% of T cells expressing both antigens. Among the 15% of nylon wool-nonadherent cells that did not express either PT4 or PT8, 5% were surface Ig+. In functional assays, cells that proliferated to PHA, hen egg lysozyme, and alloantigens were blocked by PT4 and were eliminated by PT4 and complement. PT4 and complement had no effect on CTL effectors or precursors. Reciprocally, PT8 blocked and PT8 and complement eliminated CTL effectors and precursors, but had no effect on proliferative responses. These results indicate that PT4 and PT8 define the porcine helper and cytotoxic T cell subsets, respectively.