Virus plaque-forming cell assay with vesicular stomatitis virus (VSV), which had been originally introduced by Bloom and his colleagues as a tool for the enumeration of activated lymphocytes, was first applied to the immune response of mice to a widely used antigen, i.e. sheep red blood cells (SRBC). When spleen cells taken from mice previously primed with SRBC were cultured in the presence of the antigen, lymphocytes capable of replicating VSV (antigen-induced virus plaque-forming cells, Ag-V-PFC) were generated in the culture. They seemed to appear as early as 1 day of culture, and the peak was attained by the 2nd day. Most of Ag-V-PFC belonged to T-cell population, since 80-90% of Ag-V-PFC was killed by the treatment of cultured cells with anti-thymocyte serum plus complement. In vitro generation of Ag-V-PFC seemed to be highly cross-reactive (about 40%) with a related antigen (horse red blood cells). Ag-V-PFC detected in the present experiment may not represent helper T cells, effector T cells, or their precursors because of the following: (a) The generation of Ag-V-PFC was completely suppressed by the addition of anti-SRBC mouse serum in the culture, though the helper activity was apparently augmented by the same treatment. (b) Development of Ag-V-PFC was almost completely suppressed by the pretreatment of mice with cyclophosphamide 2 days before immunization, by which delayed-type hypersensitivity (DTH) was markedly augmented. (c) After the immunization of mice, Ag-V-PFC began to develop just when the level of DTH declined, at which point helper activity of the spleen cells also diminished. A possible role of Ag-V-PFC in the immune response was discussed.