Antigen-specific helper T cells are required in the generation of cytotoxic T cells from thymocyte precursors. We have demonstrated that these alloantigen-specific helper cells can be generated in vitro and that both the quantity and quality of the helpers appear to be superior to the help obtained from unprimed spleen cells. Optimal helper cell activity is produced at day two of culture when CBA splenic helper precursors are stimulated by irradiated allogeneic spleen cells. Helper cell precursors are antigen-specific cells which cannot be instructed to express forbidden receptor specificities and bear theta antigen on their surface. The helper effectors are radioresistant, theta-bearing, and antigen-specific cells.

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