Malignant mesotheliomas were induced in the rat peritoneum by a single injection of chrysotile or crocidolite asbestos fibers. The immediate toxicity of the fibers was noted in both groups of animals, producing approximately 40% mortality, within 8 days after the injection associated with acute peritonities. Tissue reactions to these two types of asbestos were significantly different. Crocidolite fibers were easily seen by light microscopy, in the tissue sections throughout the period of study, and they produced foreign-body giant cell granulomas. However, giant cells were not seen in chrysotile granulomas, and the asbestos fibers were only seen by electron microscopic study. They appeared to be coated by a protein-like substance. During earlier stages of tumorigenesis, the epithelioid and/or mixed cell type mesotheliomas seemed to have no specific relationship to granulomas, but pure spindle cell tumors were seen to develop in close relationship to granulomas, and they appear to be fibrosarcomas. Electron microscopic and histochemical methods were used to define the morphologic characteristics of the tumor cells. The formation of hyaluronic acid was found in cells of the epithelioid type, contrasted with extracellular accumulation in the spindle cell tumors.