Human monocytes exposed to PHA-leukocyte-conditioned medium for 24 hr acquire markedly enhanced ADCC against antibody-coated human erythrocytes. Confluent monolayers of these activated monocytes were found to bind several fold the number of 51Cr-labeled antibody-coated erythrocytes as compared to monolayers formed from control monocytes (uncoated erythrocytes were not bound). The stoichiometry of this reaction indicated that activated monolayers bind approximately 3-5 erythrocyte targets per effector monocyte, whereas control monolayers bind less than 2. Bound target cells remain intact, affixed to the monocyte surface for up to 3 hr at 25 degrees C (they were not phagocytized); incubation at 37 degrees C resulted in both target cell adherence and 51Cr release suggesting that a proportion of target cells were lysed. A substantial fraction of target cells bound at 25 degrees C were lysed (activated greater than control monocytes) if the monolayers were warmed to 37 degrees C. These results indicate that target cell target cell binding can occur at both 25 and 37 degrees C, but lysis of bound targets requires incubation at 37 degrees C. Using this temperature distinction to examine binding and lysis independently, it was found that binding was abrogated by IgG Fc fragments, but could occur over a broad pH range (5-8), and in the presence of 2-deoxy-D-glucose, colchicine, and sodium azide. Lysis of bound targets, on the other hand, was blocked by inhibitors of glycolysis, microtubule/filament organization, cellular respiration, and serine esterase activity, as well as EDTA and low pH; lysis was unaffected by scavengers of extracellular H2O2 and superoxide radicals.