The influence of different degrees of avitaminosis B6 in mice on the cytolytic activity of T-lymphocytes measured by the amount of Na2Cr51O4 released from the lysed target cells was studied on a model of primary immune response in a mixed lymphocyte culture in vitro. Keeping of the animals for 3 weeks on pyridoxine-free diet failed to influence the capacity of lymphocytes to proliferate in vitro and their cytolytic activity. In animals on pyridoxine-free diet for 45 days the amount of pyridoxal 5(1)-phosphate in the spleen decreased by 55% in comparison with control. Lymphocytes obtained from these animals and cultivated in vitro had a markedly decreased capacity to 3H-thymidine incorporation into DNA in response to alloantigen. The cytolytic activity of these lymphocytes also diminished. The capacity of various pyridoxine forms to restore T-lymphocyte functions disturbed by avitaminosis B6 was studied.