AbstractThe ability of cultivated mouse peritoneal macrophages (MΦ) to release superoxide anion (O2−) after repeated stimulation by phorbol myristate acetate (PMA) or serumtreated zymosan (STZ) has been studied. After a maximal first stimulus bacillus Calmette‐Guérin (BCG)‐activated MΦ released high levels of O2−, 2‐fold more than thioglycollate‐elicited MΦ and the response ceased within 4 h. Both populations either responded again to a second challenge or displayed a refractory state which varied in duration and selectivity. Desensitization by STZ pretreatment was transient and selective whereas PMA could render MΦ refractory for 3 days to PMA alone or to both agents, depending on the amount of PMA used and the conditions of stimulation. PMA induced a selective loss of specific saturable receptors for [3H]phorbol dibutyrate, a closely related agent, and receptor activity recovered with the ability to release O2−. Loss of receptors did not account for concomitant loss of the response to STZ after nonselective deactivation. Such MΦ were fully viable and able to endocytose various soluble and particulate ligands vigorously, but without stimulation of the hexose monophosphate shunt or release of O2−.Our studies indicate that MΦ activities can be profoundly altered by prior stimulation, that specific receptors play a role in ligand‐induced desensitization and that agents such as PMA can selectively eliminate the cells′ ability to generate