2,3-Dihydroxypropanal is a minor component of normal carbohydrate metabolism; apparently both isomers of the racemic compound (D, L-glyceraldehyde) are metabolized in liver. In mice, single doses of 0.4 g/kg dihydroxypropanal reduced both sarcoma and carcinoma growth by more than 90%. Single doses also increases longevity of mice with L-1210 and P-388 leukemias by +48–60%. The responses of these two murine cancers in vivo appear similar to drug responses of human cancers. Mice of both sexes demonstrated no significant inhibition of blood-forming elements in femoral marrow following 1.5 g/kg doses of dihydroxypropanal, a circumstance rarely observed with drugs which inhibit cancer. Single large doses of dihydroxypropanal failed to effect either significant fluctuations in quantities of circulating erythrocytes, eosinophils or lymphocytes (although they induced a slight transient elevation in circulating neutrophils) or in quantitites of marrow erythroblasts, erythrocytes, lymphocytes, neutrophils or myeloid precursor cells. Intraperitoneal doses of 1.5 g/kg dihydroxypropanal produced gut irritation, but elicited no signs of gross or microscopic pathology in cardiac, renal, adrenal, testicular, hepatic, pneumal or splenic tissues. In mice, dihydroxypropanal may preferentially inhibit cancer growth without concomitantly damaging many normal tissues usually sensitive to drugs which inhibit cancer growt