Several proteins, glycoproteins, and iodoproteins are secreted from human polymorphonuclear leukocytes during phagocytosis of inert latex particles. The amount of 125I-labeled proteins increases during 10 to 60 min of incubation. The 125I-iodoproteins secreted into the incubation medium during the phagocytosis were first separated on Sephadex G-150 column and then characterized by column chromatography on Sepharose 4B, Sephadex G-200, and G-100. Three 125I-iodoproteins were found with the molecular weights of 580,000, 100,000, and 22,000. The molecular mass of 15 protein subunits calculated after sodium dodecyl sulfate-polyacrylamide gel electrophoresis ranged from 11,000 to 86,000 daltons. Four of the protein subunits were labeled with 125I. Their molecular weights were 63,000 to 69,000, 44,000 to 49,000, 22,000, and 11,000. In addition to iodoproteins, several 125I-labeled peptides and compounds were found by column chromatography on Bio-Gel P-2 and P-10. The 125I-iodoproteins are not secreted from resting or boiled granulocytes, and their production is 92 to 98% inhibited by 1 mM KCN or 1 mM sodium azide. The double-labeling technique with [125I]- and [131I]-iodine suggests that the iodoproteins formed in the phagocytosing granulocytes or secreted into the incubation medium are not identical.