Infection of the mouse peritoneal cavity by bacillus Calmette-Guérin (BCG) markedly alters the surface properties of the macrophages induced, compared with cells obtained from uninfected control animals or after injection of thioglycollate broth. Quantitative binding assays with radiolabeled ligands or antibodies showed that BCG-activated peritoneal macrophages (BCG-PM) expressed one-fourth or less receptor activity for mannose-terminal glycoconjugates as well as reduced levels of Fc receptors and of antigen F4/80 compared with nonactivated macrophages. Endocytosis mediated by mannose-specific receptors was reduced in parallel. In contrast, surface Ia antigen was increased threefold in the same adherent cell population. Radioautographic analysis confirmed that greater than 80% of adherent cells still expressed low levels of the macrophage-specific mannosyl receptor and antigen F4/80, and that I antigens had been induced on 64% of macrophages rather than on other cells. Control experiments established that only the BCG-PM macrophages released H2O2 after stimulation with phorbol myristate acetate, whereas both BCG-PM and thioglycollate-induced macrophages produced superoxide anion and plasminogen activator. The BCG-PM were viable, secreted normal levels of lysozyme, and displayed a stable phenotype after cultivation for 60 h. Inhibitors of oxygen products, prostaglandins, and proteases did not alter reduced endocytosis by BCG-PM. These studies indicated that expression of macrophage surface markers is reversed by BCG-activation, and that their known enhanced ability to lyse target cells extracellularly is associated with decreased endocytosis via specific receptors. Whether these changes are a result of an altered cell population or of modulation of selective surface properties is not known.