Most of the T cells participating in the primary immune response are rapidly eliminated, but small numbers of these cells survive and differentiate into long-lived memory cells. Information on the life history of memory cells can be obtained by studying the component of memory-phenotype T cells found in normal animals; these cells are presumed to represent memory cells specific for various environmental antigens. For CD8+ cells, in vivo exposure to viruses and certain other infectious agents causes a large proportion of memory-phenotype (CD44hi) cells to enter the cell cycle. In this situation, stimulation of CD44hi CD8+ cells does not seem to require T-cell receptor ligation and appears to reflect release of various cytokines, especially type I interferon. The capacity of infectious agents to induce non-antigen-specific stimulation of T cells may play a role in boosting the survival of memory cells and perhaps also in providing an adjuvant function during the primary response.