The macrophage is omnipresent in mammalian lungs yet poorly studied. As we become more interested in the biochemistry of the lung and especially of environmental assaults upon it, we need a better knowledge of the scope of metabolic activity of these cells. The lung as the brain is made up of metabolically distinct cells which are difficult to separate. Macrophages are separable and therefore should be studied independently so that reactions of the total lung in vivo can be better understood. Phagocytosis and digestion are the principal functions of macrophages. Phagocytosis is a highly energy dependent system. Gee et al present evidence for hydrogen peroxide metabolism during phagocytosis and concomitant regulation of gluocose metabolism. Regulation of glucose metabolism via glutathione peroxidase and reductase influence on the effective oxidation state of the nicotinamide adenine dinucleotide phosphate (NADP) pool is proposed. Certainly the potential role of H2O2 in bactericidal