SUMMARYTwo nonphagocytic effects of macrophages on antibodycoated lymphocyte target cells were studied: antibody-dependent cell-mediated cytotoxicity (ADCC) and abrogation of complement lysis (ACL) by nonphagocytic mechanisms. Macrophages did not mediate ADCC, and actually inhibited K cell-mediated ADCC, presumably by direct interactions with the antibody-coated target cells. These interactions were studied in the ACL assay previously described, in which macrophages, antibody, and51Cr-labelled target cells were incubated for 1 hr. Then complement lysis (as measured by51Cr release) was performed to assess the status of the target cells. The nonphagocytic component of ACL could be distinguished from the phagocytic component by the addition of a second antibody during the complement lysis phase. This procedure revealed that some of the target cells which were resistant to the original antibody were susceptible to lysis by a second antibody and were therefore not phagocytized. Such cells had apparently been stripped of their antibody and the associated antigen by the macrophages. In support of this interpretation, specific antigen alterations were demonstrable on these stripped cells under certain conditions. These alterations were produced more consistently when the macrophages were less than maximally stimulated, and were detected better by guinea pig complement than by rabbit complement. The mechanism of stripping may involve inactivation, redistribution, or removal of target cell-bound antibody by the macrophages. Possible in vivo roles for the stripping mechanism, for example, in the enhancement of tumours or allografts, are discussed.