The anti-tumor activity of amygdalin (NSC 251222), commercially known as Laetrile, was investigated using P388 lymphocytic leukaemia and P815 mast-cell leukaemia in BDF1 mice. Doses varying from 200 mg/kg to 2,000 mg/kg were used following the days 1 and 5 and days 1, 5 and 9 schedules. Despite treatment with high doses of amygdalin there was no prolongation in the life-span of mice bearing either P388 or P815 tumor.