A comparison between the cytotoxicity and the anti-tumoral activity of patulin and patulin-cysteine adducts has been conducted. In vitro assays using L 1210 and P 388 cells showed that patulin-cysteine adducts had less cytotoxic activity than patulin (IC50 was 4 fold of IC50 patulin). In vivo, cysteine not only reduced the toxicity of patulin but also reduced its antitumoral activity against L 1210 and P 388 mouse leukemias (respectively of 25.7% and 46.6% with 3 mg.kg-1.d-1). By addition on patulin at the 4 or 7 position, cysteine might interfere with the lactone function and the -CH2- in in position 6 which are responsible for the antitumoral activity.