Recent studies suggest that the generation of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) or superoxide (O2-) may play a role in monocyte antibody-dependent cytotoxicity (ADCC). We studied ADCC by normal human monocytes, and monocytes from chronic granulomatous disease (CGD) patients, cells unable to generate ROS, toward anti-D sensitized human red cells (RBC) and an antibody sensitized lymphoblastoid cell line (CEM) by 51Cr release. The effects of hypoxia, scavengers of ROS, and the activity of the hexose monophosphate shunt pathway (HMPS) were examined. We found that monocyte HMPS activity increased two to threefold during ADCC toward RBC but was unchanged during ADCC toward CEM cells. Hypoxia decreased lysis of RBC targets by 80% but did not affect lysis of CEM cells even though hypoxia markedly decreased monocyte HMPS activity. Monocytes from CGD patients had impaired lysis of RBC but lysed CEM cells normally. We could not, however, demonstrate protection by scavengers of ROS. We conclude that monocyte ADCC involves two independent mechanisms: a nonoxidative mechanism active in the lysis of CEM cells, and an oxidative mechanism that may involve an unidentified ROS activated during ADCC toward RBC. The activation and possible interaction of these two mechanisms is determined by the nature of the target cell and sensitizing antibody.