Human lymphocytes treated with neuraminidase show cytotoxicity reactions with a larger number of alloantisera against human leukocyte antigens than do untreated lymphocytes. In general, untreated thymus cells from the lymphocyte donor give reactions comparable to those of the neuraminidase treated lymphocytes with both alloantisera and sera which fail to react with untreated lymphocytes. However, certain sera show clear distinctions between lymphocytes and thymocytes, some reacting only with thymus cells and others only with lymphocytes. The cytotoxic reactivity of lymphocytes and thymocytes correlates with the amount of sialic acid released from the cells by neuraminidase treatment in that thymocytes release less acid than cells from spleen, adenoids, tonsils, or peripheral blood.