In recent years the bactericidal and cytotoxic actions of phagocytes and particularly neutrophils have been attributed mainly to O2-dependent systems that depend on the accumulation of chemically reactive derivatives toxic to the bacteria. This view has been reexamined in the light of new observations on the properties and potency of O2-independent bactericidal proteins that have been purified from neutrophils. These proteins include several enzymes as well as a granule-associated protein that acts specifically against certain gram-negative bacteria. Exposure of susceptible bacteria to this latter protein produces three effects: loss of ability to multiply, a discrete increase in permeability of the outer membrane of the envelope, and activation in the bacterial envelope of degradative enzymes that act on phospholipids and peptidoglycan. It is concluded that effective antimicrobial activity rests on the coexistence of O2-independent bactericidal proteins that are highly specific for certain microbial species and O2-requiring systems that nonspecifically attack all cells.