Antibody to human lymphotoxin was elicited in rabbits immunized with partially purified supernatants of human spleen cells stimulated by phytohemagglutinin; Anti-LT activity, assayed by neutralizing soluble LT toxicity to indicator mouse L cells, is confined to the immunoglobulin fraction of the serum; it cannot be removed by adsorption to L cells, human fibroblasts or lymphocytes, or to human serum. Anti-LT activity is removed by immunoadsorption to Sepharose conjugated with goat antisera to rabbit 7S immunoglobulin; partial recovery of anti-LT activity is effected by elution with high molarity acid buffer. The amounts of antisera necessary to effect neutralization are linearly proportional to LT concentration. Anti-human LT neutralizes the toxicity of supernatants of human lymphocytes activated by other, non-PHA mitogens (antigens to which the lymphocyte donors are sensitive) and allogeneic lymphocytes in mixed leucocyte culture. However, anti-human LT is specific for human LT as it will not neutralize guinea pig or rat LT.