An International Agency for Research on Cancer (IARC) committee recognized aerosol of sulphuric acid as a human carcinogen on the basis of epidemiological studies. No experimental studies on the carcinogenicity, either of sulfuric acid aerosol or of sulfuric acid itself was available. Our aim was to determine whether sulfuric acid is a causal or modifying factor in carcinogenesis, especially in the respiratory tract. We used two species of laboratory animals (both sexes) 315 Wistar rats and 219 CBAxC57Bl mice in a long term experimental study. The rats were treated with sulfuric acid (maximal tolerated doses, by chronic intratracheal instillations or by gastric intubations) and/or benzo(a)pyrene (by intratracheal instillations). The mice were treated with sulfuric acid (by chronic gastric intubations) and/or urethane (by intraperitoneal injections). We observed the animals throughout their lives and performed gross and microscopic examination of all organs. The results of the first year of study did not provide clear evidence either for sulfuric acid carcinogenicity or for co-carcinogenicity. However, in the second year tumors appeared in those organs where sulfuric acid acted directly. A modifying (stimulating) effect of sulfuric acid on carcinogenesis induced with benzo(a)pyren was observed in rats. Sulfuric acid did not influence lung carcinogenesis induced with urethane in mice.