Summary.1. Analyses of healthy leaves and of leaves infected with mosaic disease (N. tabacum) show that the diseased leaves contain higher proportions of nitrogen and lower proportions of carbohydrate, these differences being apparently increased during photosynthesis.2. In leaves kept in the dark for 68 hours, carbohydrate losses are similar but fall chiefly on insoluble carbohydrate in healthy leaves and on disaccharides in the diseased leaves.3. In diseased leaves, protein breakdown is retarded and insoluble substances are less readily hydrolysed.4. The diseased leaves resemble older leaves in their metabolism.