The carcinogenicity of nickel subsulfide, Ni3S2, for respiratory tract epithelium was studied in heterotopic tracheal transplants with doses of 1 and 3 mg Ni3S2 per trachea. Chemical determinations indicated that Ni3S2 persisted in the tracheas for seven to nine months. Ni3S2 showed marked toxicity for mucociliary epithelium, resulting in widespread atrophy and focal epithelial necrosis during the first two months of exposure. The submucosa showed mononuclear infiltration and signs of fibroblastic and capillary proliferation. Tumor studies indicated that Ni3S2 can induce carcinomas in tracheal epithelium. The carcinoma incidence was 10% at 1 mg and approximately 1.5% at 3 mg. The higher dose produced a 67% incidence of fibro- and myosarcomas. The data suggest that, compared to some carcinogenic polycyclic hydrocarbons, Ni3S2 may not be a strong carcinogen for the epithelium of conducting airways. The data are discussed in light of other experimental studies and of epidemiological findings on respiratory tract cancers in nickel workers.