Tbe distribution ofthe functional subsets ofporcine T cells, the cytolytic/suppressor (Tc/s) and the helper/inducer (Tb/i) cells was studied in cryostat sections ofthymus, Iymph nodes, tonsils, Peyer's patches, spleen and liver using the indirect immunoperoxidase technique. Tbree murine monoclonal antibodies (mAb) were used. Tbe mAb 8/1 reacts with an antigen present on all T cells and on cells of the myeloid Iineage; the antigen has not yet been characterized biochemically. Tbe mAb 295/33 (antiT8) binds to the porcine T8 antigen and defines the Tc/s subset, while mAb PT -4 (anti-T4) detects the porcine T4 antigen and defines the Tb/i subset. Practically all thymocytes were stained by mAb 8/1. Tbe majority of corticaI thymocytes apparently co-expressed T8 and T 4, whereas distinct fractions of medullary cells were labelIed by either anti-T8 or anti-T4. In peripherallymphoid organs all three mAb reacted with cells in the thymus-dependent areas and with cells scattered in the lymphoid follic1es. In Iymph nodes, tonsils and Peyer's patches, anti-T8 and anti-T4 each labelIed approximately half of the cells stained by mAb 8/ I. In the periarteriolar lymphoid sheath of the spleen, anti-T 4 labelled more cells than did anti-T8. The reactivity of mAb 8/1 with the Kupffer cells of the Iiver demonstrated the expression of the 8/1 antigen on cells of the monocyte lineage. Tbe T8 and T4 antigens could not be detected in acetone-fixed and paraffin-embedded sections, while the antigen recognized by mAb 8/1 remained preserved. Altogether, despite an inverted microanatomical structure ofporcine Iymph nodes, the frequency and distribution ofT8+ and T4+ cells in thymusdependent areas proved to be similar to those found in other species.