The immune system of swine is unique in that the expression of CD4 and CD8 antigens defines four subpopulations of resting extrathymic T lymphocytes. Beyond phenotypic differences to other species, porcine T lymphocytes, particularly when derived from infected animals, are known to show high non-specific cytolytic in vitro activity. Here we describe the putative porcine CD6 antigen (workshop CD6; wCD6) which enables a phenotypic separation of T lymphocytes responsible for major histocompatibility complex (MHC)-restricted and non-MHC-restricted cytotoxicity. The putative porcine CD6 analogue, wCD6, a protein with a molecular mass of 110 000, shows high specificity for T lymphocytes and is neither expressed on B lymphocytes nor on cells of the myeloid lineage. In the extrathymic T-lymphocyte compartment wCD6 characterizes two T-lymphocyte fractions: wCD6(+) T lymphocytes including both CD4(+) T-helper cell subpopulations (CD4(+)CD8(-) and CD3(+)CD8(+)) and within the CD4(-)CD8(+) fraction cells with high CD8 antigen density. In contrast the CD4(-)CD8(-) gamma/delta T-cell receptor (TCR) subset and CD4(-)CD8(+) cells with low CD8 antigen density are included in the wCD6(-) T-lymphocyte fraction. Functional studies with separated wCD6 fractions revealed that the wCD6(-) cells can be characterized by spontaneous and non-MHC restricted cytolytic activity, whereas the wCD6(+) T lymphocytes are responsible for MHC-restricted T-cell functions. Thus, the porcine wCD6 is an important antigen to discriminate between MHC-restricted and non-MHC-restricted cytotoxicity.