Interleukin (IL)-10 generally is considered a macrophage deactivating factor and thus inhibits the cell-mediated responses against intracellular parasites. We evaluated the in vitro effects of IL-10 on three different parameters of macrophage function. We found that IL-10 inhibited gamma interferon (IFN-γ) priming for enhanced O2− release of mouse bone marrow-derived macrophages but had opposing effects on NO2− secretion according to the sequence of the treatment with IL-10 and the agonists of NO2− secretion. Likewise, IL-10 was able to induce a small but consistent degree of bacteriostasis of Mycobacterium avium and, also, to inhibit the bacteriostasis induced by IFN-γ. Thus, we show that, according to the timing of exposure of macrophages to stimulating and inhibiting cytokines and agonists of their functions, IL-10 shows different effects on macrophage function.