T lymphocytes obtained from pigs infected with a lethal dose of classical swine fever virus were analysed for phenotypic changes in the composition of T-cell subpopulations and for alterations in their immune responsiveness in vitro during the course of disease. Viral antigen detected in all subpopulations and the selective depletion of CD4- CD8- gamma/delta T cells showed that peripheral blood T lymphocytes were affected in the terminal stage (14-19 days post-infection) of classical swine fever whereas no implications for T lymphocytes were obvious during the first 10 days after infection. Furthermore, a depletion of CD1+ CD4+ CD8+ "common thymocytes" was characteristic for the infected animals. Studies on immune functions of peripheral T lymphocytes revealed an abrogation of cellular immune responses as early as 3-5 days after infection and thus before detection of viral antigens in these cells. The data suggest that early immunosuppression represents a crucial event for the manifestation of classical swine fever.