Several laboratory animal studies have been up to July 1993 published with the aim to investigate possible carcinogenic or cocarcinogenic effects of magnetic fields. No large-scale study of complete carcinogenicity, considering tumor as an endpoint, has been performed. Nor has any study on tumor initiation only been performed. In studies designed for tumor promotion, small parallel series for complete carcinogenicity have also been included in some experiments. These small series do not permit an evaluation of the possible complete carcinogenicity or noncarcinogenicity of magnetic fields. Studies designed to illuminate the possibility of a tumor-promotive effect of magnetic fields seem not to support the hypothesis that continuous magnetic fields act as promoters in rats or mice. Observations on skin tumors in mice seem to indicate a possible copromotional effect of a 2-mT 60-Hz magnetic field and a simultaneous treatment with a known skin tumor promoter in the sensitive Sencar mouse strain.