The current status of T-cell subset involvement in viral immunity is summarized for experimental studies in mice. The immunobiology of the normal host response is discussed, with particular reference to lymphocytic choriomeningitis (LCM) and influenza. The general impression is that CD8+ cytotoxic T lymphocytes, CD4+ TH1 cells, gamma interferon, and IL-2 are of major importance, with these different components of the immune system interacting to promote an optimal response. However, experiments with a variety of virus systems indicate that there is considerable plasticity, at least in young, adult mice. Other mechanisms often compensate if a key lymphocyte subset is absent throughout the development of the immune response. Influenza-infected mice depleted of either CD4+ or CD8+ T cells clear virus and recover, though the latter may not be true for the elimination of LCM virus. Emerging information on the involvement of gamma delta T cells in viral pneumonia is summarized, but there is as yet no understanding of the biological significance (if any) of these lymphocytes in viral immunity. The point is made that alpha beta T-cell memory to viruses is long-lived, and the need for antigen persistence to maintain such memory is questioned.