Fischer rats in five experimental groups were given a single injection of penicillin suspension containing carcinogenic dust (2.5 milligrams), alone or in combination with equimolar amounts of aluminum, copper (7440508), chromium (7440473), or manganese (7439965) dusts. Rats in five control groups were treated identically, except that in the intramuscular nickel-subsulfide (12035722) dust was omitted. After 24 months, the incidence of sarcomas at the injection site was 63 percent in the group that received the combination of nickel-subsulfide and manganese dusts, compared with incidences of 96 to 100 percent in the groups that received nickel-subsulfide alone or in combination with aluminum, copper, or chromimum dusts. No sarcomas occurred at the injection site in control groups that did not receive nickel-subsulfide. The finding that the addition of equimolar amounts of manganese dust to nickel-subsulfide dust significantly depresses nickel-subsulfide induced tumorigenesis provides an experimental system for investigations of metal interactions in carcinogenesis.