Six basic isoenzymes, E1 to E6, of horseradish peroxidase were isolated and purified by CM-Sephadex column chromatography, and were then crystallized. The crystalline basic isoenzymes had a lower molecular weights than the neutral isoenzyme C, but the molecular weights of their protein moieties were similar to that of the latter. The lower molecular weights of the basic isoenzymes were thus essentially due to smaller contents of carbohydrate. Of the six isoenzymes, four isoenzymes, E3 to E6, had extremely high pI values of over 12 and appreciably low contents of carbohydrate, 0.8 to 4.2%, whereas the isoenzymes E1 and E2 contained relatively large amounts of carbohydrates, 12.8 and 14.1%, respectively, and had lower pI values, 10.6. The amino acid compositions of the basic isoenzymes were different from one another, and the differences suggested that the isoenzymes were allelic. The relatedness of the amino acid compositions of these isoenzymes is discussed and the compositions are compared with those of other peroxidases. Finally, one of the six isoenzymes, E4, was obtained as large single crystals suitable for X-ray structural analysis, and the preliminary crystallographic data are presented.