[en] Benzo-5,6 flavone is a chemical compound that is not known to be carcinogenic but which is a strong inducer of microsomal enzymes for the cells of pulmonary tissue. Several groups of animals received 6000WLM of radon over two months. This dose produces lung cancers in over 50% of the animals with a latency time of more than 15 months after the start of treatment. The first group of 20 rats received 25mg/kg of benzoflavone in weekly intraperitoneal injections for 13 weeks. The first injection was given three months after the end of the radon exposure. The first lung cancers appeared even before the end of the treatment and all the animals showed cancers within two months. All the tumours were of the epidermoid, multifocal type and invaded the lung. The animals that had been exposed to radon only did not show any lesions by this date. The second group of eight rats received the same benzoflavone treatment, but beginning one year later, i.e. 16 months after the start of the radon exposure. No differences were observed between the pulmonary lesions in these animals and those that had received radon only. The animals that were treated with benzoflavone only showed no pulmonary lesions. In parallel with the experiment on the cocarcinogenic effects of radon and benzoflavone, microsomal enzyme doses were given to other animals that had received the same radon dose but at different times. The results should help to explain the cocarcinogenic effect of benzoflavone. (author)