The highly cationic proteins of human granulocytes, whose electrophoretic mobilities toward the cathode are faster than that for lysozyme, were isolated from the cytoplasmic granules of leukocytes, obtained from patients with chronic myeloid leukemia. The granule extract was subjected to chromatography on Sephadex G-75 and E-aminocaproic acid-Sepharose ion adsorbant followed by preparative electrophoresis on agarose. Seven cationic protein components were identified, and five of these were obtained in a pure form. One group of cationic proteins, including components 1-4, exhibited molecular weights in the range 25,500-28,500, almost identical amino acid composition, and complete immunologic identity. Another group of proteins, including components 5-7, exhibited molecular weights in the range 21,000-29,000 and also showed complete immunologic identity; amino acid analysis performed on component 5 indicated a different amino acid composition from that of components 1-4. Cationic proteins with similar electrophoretic mobilities and immunochemical identities were also detected in granule extracts of granulocytes from healthy individuals. The proteins isolated from human granulocytes have a higher molecular weight and a lower content of basic amino acids than the cationic proteins with antibacterial and permeability-increasing properties previously demonstrated in rabbit polymorphonuclear granulocytes.