A cytotoxic factor for rat lymphocytes present in cobra venom is described and partially characterized. This cytotoxic factor (CF) was isolated by gel filtration on Sephadex G-100 from anticomplementary cobra venom factor (CVF) that had been previously partly purified by DEAE chromatography. The CF elutes from G-100 later than does the anticomplementary CVF. Estimates indicate a molecular weight of about 13,000. While rat lymphocytes are very sensitive to this factor as indicated by a decrease in capacity to exclude trypan blue, erythrocytes and peritoneal leukocytes are more resistant. The activity of CF is temperature dependent. Studies of the local graft-vs-host reaction revealed an inhibitory influence of the partially purified DEAE-CVF that was attributable to the new CF of cobra venom. The role of the two CVF in GVH disease and other cellular immune functions is discussed.