Two lines of transgenic mice carrying the gene for granulocyte-macrophage colony-stimulating factor (GM-CSF) produce vastly increased numbers of macrophages with abundant foamy cytoplasm resembling classical activated macrophages. Cells from both lines were negative for myeloperoxidase, a bactericidal enzyme found in monocytes as well as neutrophils, but not mature macrophages. Cells from the so called 'male line' produced greatly increased levels of oxygen degradation products in response to phagocytosis, compared with cells from the 'female line' or from normal littermates. The ability of the cells to phagocytose and lyse the intracellular bacterium Listeria monocytogenes was tested in vitro using radiolabelled organisms. Although the cells from transgenic mice were more highly phagocytic than cells from normal littermates, cells from either line were no more efficient than normal at lysing the bacteria they had phagocytosed. Nevertheless, because of the high phagocytic rate, more bacteria were exposed to lysis in the cells of transgenic mice, and the final outcome was a higher rate of bacteriolysis.