On phagocytosing a microorganism, the neutrophil (polymorphonuclear leukocyte, PMN) consumes oxygen at a sharply elevated rate1. The oxygen is used to kill the microorganism, presumably being used to produce a potent oxidizing agent or agents. Candidates for these bactericidal agents are singlet oxygen, hydroxyl radical and chlorinating agents (that is, species containing 'active' Cl in a formal +1 oxidation state: HOCl, Cl2, N-chloroamides, and so on)1-5. We now report a semiquantitative assay for PMN-generated active chlorine based on its trapping with 1,3,5-trimethoxybenzene (TMB). Using this assay, we have found that at least 28% of the oxygen consumed by stimulated normal human PMNs is converted to active chlorinating agents.