The luminol-enhanced chemiluminescence of zymosan-stimulated polymorphonuclear leukocytes is continuously diminished in the presence of increasing amounts of human serum albumin (from 1 to 30 mg/ml). HSA competes with luminol for hypochlorite as shown by adding sodium hypochlorite to luminol solutions containing HSA. Titration of HSA with NaOCl affects firstly the sulfhydryl and probably the thioether groups and then the amino groups. Coincubation of zymosan-stimulated polymorphonuclear leukocytes and HSA at a ratio corresponding to normal values of healthy men causes a continuous oxidation of sulphur containing functional groups in HSA.