An enzyme preparation from fresh tomato fruit catalyzed the degradation of l-leucine. The reaction products were separated as their 2,4-dinitrophenylhydrazones and analyzed by TLC. α-Ketoisocaproic acid, the keto acid corresponding to l-leucine, was identified. The conversion was confirmed by studies with l-leucine-U- 14C as substrate. Enzyme activity was found to occur predominantly in the supernatant fraction and to decrease with ripening of fruit.