Mutagenic activities of cigarette smoke condensate were assayed in the presence of S-9 Mix using Salmonella typhimurium TA 98. The results were examined in relation to chemical data of tobacco leaves. Among the nitrogenous constituents examined, the contents of total nitrogen and protein nitrogen and the soluble nitrogenous fraction were positively and significantly related to an increase in mutagenic activity of the smoke condensate, whereas nicotine and nitrate were not important in contributing to mutagenic potency of such condensates. The age of tobacco leaves influenced the mutagenic potency of the condensate, which was lowest in leaves from the lower stalk position and increased with ascending leaf position on the stalk. Smoke condensate from tobacco with higher sugar content resulted in lower mutagenic activity. The present results, together with the previous study on the mutagenicity of the amino acid pyrolyzates, suggest that potent mutagens in cigarette smoke condensate are nitrogen-containing compounds, which may be formed from proteins and amino acids during the burning of a cigarette.