When spleen cells from mice injected 3 weeks or more previously with influenza A virus (responder cells) are mixed with normal spleen cells exposed 1 h previously to influenza A virus (stimulator cells) and the mixture cultivated at 37 degree for 5-6 days, the surviving cell population contains effector T cells (Td) which can mediate delayed type hypersensitivity reactions. If infectious virus is used to prime both the donor mice and to infect the stimulator cells, the cell population also contains cytotoxic T cells (Tc). In this case, both Tc and Td have similar specificity patterns, as cells raised to one sub-type A virus are cross-reactive to other A strain viruses but not to Sendai virus. If non-infectious virus is used to immunize the donor mice, Td but not Tc are generated and these cells are specific for the sub-type A virus used in the original immunization. Both preparations of Td are Lyl+23- and require IA sharing of donor and recipient mice for transfer DTH activity to be successful. Td cells produced this way are similar to those produced in vivo except they may have different migratory properties and must be injected directly into the footpad for DTH activity to be elicited. In such transfers, H-2 restriction can be clearly demonstrated if the challenging antigen is injected into the footpad some hours before the injection of cells.