2,4-Diamino-6-(bis-2-chloroethyl)aminomethyl pteridine has been synthesized and found to be highly potent against L-1210 mouse leukemia lymphoblasts. A single dose of 5 mg/kg injected 24 h after the tumor inoculation increased the life-span of 50% of mice to more than 200%, while the other 505 of animals were cured. © 1984 Springer-Verlag.