1. 1. Lipoxygenase (linoleate:oxygen oxidoreductase, EC 1.13.1.13.) from peas was extensively purified by precipitation with (NH4)2SO4, gel filtration with Sephadex G-150, and ion-exchange chromatography on DEAE-cellulose. 2. 2. The final enzyme preparation proved homogeneous by ultracentrifugation of a 0.6% protein solution (pH 7.0) but separated into two main and narrow fractions on isoelectric focusing, pI values 5.80-5.82. 3. 3. The molecular weight, as calculated on the basis of amino acid analysis and ultracentrifugation, was found to be 72 000 and 67 000, respectively. 4. 4. Pea lipoxygenase contains 7 half-cystine residues, different from the soybean enzyme which has been reported to contain none. © 1970.