Two subclasses of CD3 associated T-cell receptors (TcR) have been described so far, consisting of either an alpha and beta chain (TcR alpha beta) or a gamma and delta chain (TcR gamma delta). Of the two subclasses, the TcR alpha beta is the one predominantly expressed on peripheral T lymphocytes of humans and rodents. TcR gamma delta T lymphocytes represent only a minor subset in these species. Among all mammalian species studied so far, swine showed the most diversified composition of the T-lymphocyte population characterized by the expression of CD4 and CD8 differentiation antigens. Besides CD4(+)CD8(-) and CD4(-)CD8(+) T lymphocytes, CD4(+)CD8(+) and CD4(-)CD8(-) T lymphocytes are prominent in the extrathymic T-lymphocyte compartment. Because of the lack of specific monoclonal antibodies (mAb), to date the porcine TcR can only be characterized with biochemical and molecular biological methods. TcR on porcine peripheral blood T lymphocytes with the phenotype CD4(+) and/ or CD8(+) are characterized as 46-48 kDa(R) heterodimers which were supposed to represent the porcine TcR alpha beta. Biochemical analyses of the CD4(-)CD8(-) T lymphocytes revealed three distinct TcR gamma delta; all are characterized by a 40 kDa delta chain but differed in their gamma chains. One gamma chain with a molecular mass of 38 kDa(R) is preferentially expressed on CD4(-)CD8(-) T lymphocytes derived from peripheral blood; another chain with molecular mass of 37 kDa(R) is evenly distributed between CD4(-)CD8(-) T lymphocytes from blood and lymphoid tissues. A third gamma chain characterized by a molecular mass of 46 kDa(R) is expressed on CD2(+)CD4(-)CD8(-) T lymphocytes enriched in lymphoid tissues. The molecular characterization of porcine TcR-constant regions revealed the existence of one alpha,possibly two beta, at least three gamma and one delta TcR-constant region isotypes, which show very high homology on amino acid level with the corresponding TcR chains of other species.