AbstractLuminol‐dependent chemiluminescence and thiol group oxidation of glutathione and human serum albumin were measured in order to demonstrate whether the inhibition of polymorphonuclear leukocyte chemiluminescence by albumin was attributable to thiol group oxidation. We have shown that:1thiol groups on glutathione and albumin are oxidized by PMNL stimulated by soluble and phagocytic stimuli;2thiol group oxidation in albumin and glutathione did not correlate with the inhibitory effects of these substances on luminol‐dependent chemiluminescence with respect to time course, magnitude, effects of known scavengers or extracellular activity. It was therefore concluded that thiol group oxidation was not the cause of albumin inhibition of luminol‐dependent chemiluminescence;3a metastable oxidant was identified after PMNL activation which was capable of oxidizing thiol groups but unable to elicit chemiluminescence form lu