The amino acid sequence of basic isozyme E5 of Horseradish Peroxidase(HRP E5) was determined by protein sequencing. HRP E5 consisted about 300 residues, and has a molecular weight of approximately 36, 000±500 dalton. The protein was rich in aspartic acid (14%), arginine (13%), and leucine(ll%). The primary structure of HRP E5 was established by sequencing its tryptic(T_1-T_19) and lysylendopeptic(A_1-A_3) peptides. The sequence homology between HRP E5 and HRP C(neutral isozyme of horseradish peroxidase) is found to be more than 66%. The highest concentration of identical residues are found on residues 29∼56, 90∼123, and 155∼173, but relatively low on 174∼271.