SUMMARY Antibodies which kill lymphocytes from other persons were found in 35 of 113 healthy subjects immunized with influenza, polio, and other vaccines. These lymphocytotoxic antibodies were characteristically more active at colder temperatures (15 C) and were IgM in nature, as were similar cytotoxins found earlier in systemic lupus, rheumatoid arthritis, infectious mononucleosis, rubella, and measles. It is suggested that a wide scale cross reactivity of microbial and human lymphocyte antigens may exist.