The importance of swine in agriculture has resulted in a substantial increase in research efforts on the swine immune system during the past few years. A better knowledge of the porcine immune system is required before improved vaccination strategies, the design of more efficient vaccines and breeding for disease resistance will be able to contribute to a reduction in the extensive economic losses caused by the disabling effects of viral, bacterial and parasitic infections. T lymphocytes play a central role in the antigen-specific immune response to the various pathogens. To detect and characterise porcine T lymphocytes, monoclonal antibodies (mAbs) were raised against different leucocyte differentiation antigens and classified for specificity in two international workshops. These mAbs have enabled detailed studies to be made on specific cell populations involved in the porcine immune response to pathogens, on T lymphocytes and on the peculiarities of porcine T lymphocyte sub-populations: extra-thymic CD4+CD8+ T lymphocytes and a substantial proportion of CD2-TCR gamma delta+ T cells. Furthermore, these reagents and the increased knowledge of the immune system have allowed studies of the interactions of T lymphocyte sub-populations with regard to different pathogens and the role which these play in infections.