A study was made of the cytotoxic and complement-fixation activity of the antisera to the cortex and white matter of the rat and human brain upon the mouse and rat thymus and bone marrow cells. The cytotoxicity test proved to be more sensitive and precise. Cytotoxins to rodent thymocytes were revealed on ly in the antisera against the human brain cortex; at the same time they were revealed both in the antisera against the cortex and against the white matter of the rat brain (much more was found in the former). The sera against the rat brain cortex lost their cytotoxicity after the exhaustion with the same antigen, but retained it when the exhaustion was carried out by the white matter.