Nuclei of murine T lymphocytes or B lymphocytes were purified and transferred into lethally irradiated whole spleen cells or B or T lymphocytes by means of polyethyleneglycol-mediated cell fusion. Transfer of lymphocyte nuclei could save the irradiated cells from cell death, and such reconstituted cells could respond to mitogens. The present study showed that nuclei of T cells could be activated in the concanavalin A-stimulated T cell cytoplasms but not in the lipopolysaccharide-stimulated B cell cytoplasms. On the other hand, nuclei of B cells were activated in the lipopolysaccharide-stimulated B cells but not in the concanavalin A-stimulated T cell cytoplasms. These data suggested that a specific interaction between cytoplasm and nucleus might exist in the activation of nuclei of each lymphocyte subset.