Myeloperoxidase (MPO) was isolated from rat peritoneal leukocytes with a yield of 51% and A430/A280 = 0.75 - 0.80, and its physicochemical properties were studied. The molecular weight of the MPO is about 150 kD. The MPO was assayed for amino acid content. We used substrate mixture containing phenol, 4-aminoantipyrine, and H2O2 to detect 10(-10) M of the enzyme. The MPO was localized in rat blood neutrophils using polyclonal anti-MPO antibodies and secondary fluorescein isothiocyanate-labeled antibodies. Immunofluorimetric assay (IFMA) was developed for quantitative measurement of the MPO. The MPO and leukocytes can iodinate BSA using NaI or thyroxine as the source of iodine.