Sarcomas in mice were induced by i.m. and s.c. administration of 63Ni- and 35S-labeled nickel subsulfide (Ni3S2), and the fate of the Ni3S2 was studied in tumors and normal tissues during carcinogenesis. Whole-body autoradiography showed a gradual loss of solubilized 63Ni and 35S radioactivity from the site of injection. There was also a loss of nonsolubilized dust particles which appeared to be phagocytized by reticuloendothelial cells in the liver, spleen, and regional lymph nodes. Microautoradiography showed that the totally dominating radioactivity within both the 63Ni3S2- and the Ni3(35)S2-induced tumors was associated with dust particles. There was no specific or excessive localization of solubilized radioactivity in the tumors or in metastases (when present). Two patterns of localization of dust particles within the tumors were observed: one with particles concentrated in a central part of the tumor and one with the particles present in the periphery of the tumor. X-ray powder diffraction of the insoluble crystalline material in the tumors indicated that a conversion of the alpha Ni3S2 to alpha Ni7S6 and beta NiS had occurred.