The conversion of iodide to a trichloroacetic acid-precipitable form (iodination) by polymorphonuclear leukocytes (PMN's) has been re-evaluated as a measure of neutrophil function. Optimum conditions are described which result in an iodination value for normal cells during the phagocytosis of zymosan of 64.1 +/- 13.2 (S.D.) nmol. per 10(7) PMN's per hour. Iodination is inhibited by agents which decrease phagocytosis, inhibit myeloperoxidase-catalyzed reactions, or degrade H2O2 and is stimulated by superoxide dismutase, an enzyme which catalyzes the conversion of the superoxide anion to oxygen and H2O2. When patients' cells and normal serum are employed, the iodinating capacity of the patients' cells is evaluated. It is low in patients with myeloperoxidase deficiency and chronic granulomatous disease, and an intermediate value was observed in a carrier of chronic granulomatous disease. When normal cells and patients' serum are employed, the iodination reaction is an indirect measure of the opsonic activity of the patients' serum. The decreased opsonic activity for zymosan of human sera deficient in the fourth or third component of complement was demonstrated in this way. Thus measurement of iodination is a convenient and sensitive screening test for cellular or humoral abnormalities of the phagocytic process.