Previous work has revealed striking differences in the peripheral T-lymphocyte compartments of swine compared to other species. The key difference is the existence of four T-lymphocyte subpopulations defined by their CD4/CD8 expression. Besides this difference in the CD4/CD8 antigen expression, we report here another unusual antigen distribution on porcine extrathymic T lymphocytes: the expression of the 63,000 MW CD5 antigen. In porcine thymus the CD5 antigen shows a biphasic antigen density, whereas the majority of thymocytes are characterized by an intermediate CD5 expression. In the extrathymic T-lymphocyte compartment, CD5 also shows a heterogeneous antigen distribution as defined by three subsets: CD5(high), CD5(dim) and CD5(-) T lymphocytes. Analyses of the CD5 expression on the four CD4/CD8-defined peripheral T-cell subpopulations revealed that all CD4(+) T lymphocytes, CD4(+) CD8(+) as well as CD4(+) CD8(-) T lymphocytes, belonged to the subset with high CD5 antigen density. The CD4(-) CD8(-) subpopulation, containing in the majority T-cell receptor (TcR)gamma delta T lymphocytes, was characterized by dim CD5 expression. The most notable difference was the division of the CD4(-) CD8(+) cytolytic T-lymphocyte subpopulation into two CD5-defined subsets: CD4(-) CD5(-) CD8(+) lymphocytes with spontaneous cytolytic activity against tumour cells, and CD4(-) CD5(+) CD8(+) T lymphocytes with major histocompatibility complex (MHC)-restricted cytolytic function. Thus, the porcine CD5 antigen is an important marker to discriminate between CD5(-) CD8(+) natural killer (NK) cells and CD5(+) CD8(+) progenitors of MHC-restricted cytolytic T lymphocytes.