Antilymphocyte antibodies were found in 51 of 83 serum specimens from Thai children with dengue hemorrhagic fever (DHF). The lymphocytotoxic activity was complement dependent, and cytotoxicity was detected in the 19S immunoglobulin M-associated serum fractions at a temperature optimum of 15 degrees C. Sera with lymphocytotoxic activity were cytotoxic to autologous as well as allogeneic lymphocytes from patients and healthy adult donors and were directed primarily against B cells, with some T cell cross-reactivity. This study suggests that infection with DHF induces predominately cold-reactive antilymphocyte antibodies in DHF patients that could potentially interact with peripheral blood cells of patients and modulate the humoral immune responses of patients during infection.