Infective influenza virus primes mice and increases at least ten-fold the level of splenic cytotoxic T-memory and precursor cells in comparison with normal mice. Intranasal virus infection or intraperitoneal injection of infective virus results in frequencies of 1-2 x 10(-4) cytotoxic T-cell precursors in spleen as determined by limiting dilution assays. With both types of immunization, T-helper cells amplifying the generation of T-killer cells are limiting, and optimal clone frequencies depend on addition of excess T-helper cells. We find that at least part of the T-helper cells amplifying the generation of cytotoxic T cells are cross reactive for the type A influenza viruses and therefore have a similar virus specificity to type A influenza-specific cytotoxic T cells (tc). Help for T-killer cells can be replaced by supernatants derived from Concanavalin A-stimulated rat spleen cells, but presence of antigen is still required to stimulate the Tc precursor or memory cells before they respond to antigen non-specific T cell-growth factor(s) present in the stimulated rat spleen cell medium.