Abstract: There are provided novel anti-leukemic epoxides which are isolated from Baccharis megapotamica. The novel compounds are characterized by a 12,13-epoxytrichothecene central ring system which is spanned by a dienic macrolide ester side chain. The active compounds are characterized by the presence of an oxygen atom in the A-ring which takes the form of either a beta hydroxy group in the 8 position together with an unsaturated bond at the 9: 10 position or, alternatively, an epoxide group lying between the 9 and 10 position. Compounds devoid of either of these groups show no anti-leukemic activity.The active compounds possess high and surprising anti-leukemic activity against P-388 lymphocytic leukemia in in vivo assays in mice.