Sera from 464 multiparous women of mixed ethnic origin and 114 Amerindian mothers (Warao of Western Venezuela), were studied for lymphocytotoxicity using a panel of B and T lymphocytes isolated from 50 to 100 individuals from the same populations. Twenty-five and 9.65% of the sera, respectively, showed activity against at least 5% of the panel. Among the women of mixed ethnic origin 10% produced antibodies directed against one or more HLA Class I antigens while 2 sera contained monospecific anti-DR antibodies; among the Warao, only 2 sera recognized HLA antigens: a Bw51 and an undetermined Cw antigen which shows linkage disequilibrium with Bw16 in both populations.