A cationic isoperoxidase, designated C₃, was isolated from Korean-radish (Raphanus savatis L.) root. Purification of the enzyme was accomplished by CM-cellulose chromatography and Sephadex G-100 gel filtration. The enzyme was a glycoprotein and its molecular weight was approximately 44,000 as determined by SDS-PAGE and Sephadex G-150 gel filtration. The optimal pH of the purified enzyme was 6.0 for guaiacol and 5.0 for H₂O₂. The Km values for guaiacol and H₂O₂ were 5.6 mM and 0.77 mM, respectively. The Km values against several naturally occurring phenolic compounds were also determined. In comparison with the anionic isoperoxidase A₂ which was previously purified from the same source, C₃ had very high Km values for scopoletin and ferulic acid.