1,2-Diformylhydrazine was administered in drinking water as a 2% solution to randomly bred Swiss albino mice for life from 6 weeks of age. As a result of treatment, the lung tumor incidence rose from 15 to 96% in females and from 22 to 82% in males. The treatment had no statistically significant effect on the development of other types of tumors. Histopathologically, the neoplasms exhibited the characteristic appearance of adenomas and adenocarcinomas of the lungs. The work is part of a series of studies aimed at revealing the relative carcinogenic potency of mono- and dialkyl-hydrazines and also their possible environmental significance as cancer causative agents.