Abstract In previous studies we have observed the production of a cytotoxic factor by the T-lymphocytes in the spleen of dengue type-2 virus-(DV) infected mice which killed normal mouse spleen cells in vitro. In the present study types of spleen cells affected in vitro by the cytotoxic factor (CF) have been investigated. It was observed that it kills most of the macrophages, one-third of T-lymphocytes and a few of the granulocytes, normoblasts and megakaryocytes. It had no effect on B-lymphocytes. The proportion of killed cells could not be significantly enhanced by retreatment of cells by additional CF. The cytotoxic activity of CF is not affected by adsorption with susceptible or non-susceptible cells. CF also kills lymphoid cells of animals of other species, viz. albino rats, rabbits, guinea pigs but had no effect on those of rooster. It had no effect on various cell cultures.