Isolated neutrophils from healthy donors were used for the isolation of four highly purified forms of myeloperoxidase as determined by spectral (A430/A280 ratio 0.80-0.87) and enzyme-activity measurements. Although the myeloperoxidases exhibited different elution profiles on cation-exchange chromatography, gel filtration indicated similar relative molecular masses. When these forms were assayed for peroxidase and peroxidase-oxidase activities with several substrates, they all exhibited virtually the same specific activities. These results suggest that possible functional differences between the enzymes may be related to differences in their sites of action rather than to differences in enzyme activity. Myeloperoxidase from a patient with chronic myeloid leukaemia also revealed a similar heterogeneity on cation-exchange chromatography. However, this myeloperoxidase contained in addition one form with a lower and one form with a higher relative molecular mass, as indicated by gel-filtration chromatography.