Swine possess an extraordinarily high number of T lymphocytes with the phenotype CD4-CD8- in peripheral blood as well as in lymphoid tissues. This subpopulation is subdivided into at least four subsets defined by the expression of CD2 and three biochemically distinct gamma/delta T cell receptors. The four subsets differ largely in their pattern of lymphoid homing in that CD2- subsets, historically referred to as null lymphocytes, predominate in the circulating pool, whereas CD2+ subsets are enriched in lymphoid tissues. Here we document the expression of all three types of gamma/delta T cell receptors by CD4-CD8- porcine thymocytes, which provides the first evidence for a thymic origination of all subsets of porcine gamma/delta T lymphocytes. The biochemical analysis shows that three distinct gamma-chains form disulfide-bonded cell surface heterodimers with a common delta-chain and that glycosylation of all chains is already completed within the thymus. Surprisingly, CD2- subsets, which are known to be enriched among thymic emigrants and which numerically predominate in peripheral blood, are underrepresented in the thymus, suggesting a high export rate.