Chronically administered butylated hydroxytoluene (BHT) can enhance the formation of carcinogen-induced lung tumors in mice, and biotransformation of BHT is required for this activity. In the present study, oxidative metabolism of BHT by liver microsomes and lung tumor promotion by BHT in three inbred mouse strains, NGP/N, A/J and MA/MyJ, were compared. The strain order shown is the order of increasing susceptibility of these mice to BHT tumor promotion which correlates with their increasing ability to produce a particular metabolite, BHT-BuOH, by hydroxylation of BHT at one of the tert-butyl groups. Chronic BHT administration, according to the treatment regimen for promotion, selectively induced the BHT oxidization pathway leading to BHT-BuOH. The results suggest that formation of BHT-BuOH leads to the tumor promoting effects of BHT. This hypothesis was tested directly by chronic administration of BHT, BHT-BuOH, and two other metabolites, 2,6-di-tert-butyl-4-hydroxymethyl phenol and 2,6-di-tert-butyl-1,4-benzoquinone to MA/MyJ mice following a single injection of urethane. The only metabolite that enhanced lung tumor formation was BHT-BuOH, and it was effective at one-fourth the effective dose of BHT. Thus both indirect and direct evidence implicates BHT-BuOH formation as an important step in the chain of events leading to promotion of lung tumors.