The relative rates of the incorporation of tracer from equimolar amounts of pyruvate-2-C14 and acetate-1-C14 into respiratory carbon dioxide, protein-bound amino acids, cholesterol and long-chain fatty acids were studied using surviving tissue slices of mouse tumors. The strains studied consisted of two spontaneous and two transplanted mammary carcinomas, two transplanted sarcomas, one lymphosarcoma and two interstitial cell carcinomas of the testis. On account of the ratios of incorporation of C14 from the two precursors into the four compounds, it was concluded that enough of the metabolic characteristics of the various tumors of each group was retained to warrant a classification on this basis.