The method described in the previous paper was used to induce secondary responses in spleen cells from CBA/H mice, pre-primed with lymphocytic choriomeningitis (LCM) virus by culturing them with LCM-infected peritoneal cells. The cytolytic effector cells thus generated have been characterized. Effector cells were sensitive to treatment with anti-theta ascitic fluid and complement. Separation procedures based on rosetting of certain categories of lymphocytes with sheep red cells through an Isopaque-Ficoll gradient indicated that effector cells lacked surface immunoglobulin and generally did not bear Fc receptors. Cytolytic activity was restricted by the H-2 gene complex. Killing had single-hit characteristics. All these results suggested that the cells from memory cultures mediating cytolysis were T cells. There was evidence for two T cell subsets, a major subpopulation directed against antigens on infected targets and a minor one directed against antigens on uninfected, H-2-compatible targets. Specificity was present at the infected cell:memory responder and killer:target levels between LCM virus (an arenavirus) and ectromelia virus (a poxvirus).