Significant variations in the protein and carbohydrate contents of bacterial cells and of mycobacterial fractions occurred during the growth cycle of three strains of Mycobacteria (H37Rv, H37Ra, and BCG).In whole bacilli or their water-soluble extracts, a peak of proteins occurred early during the exponential phase of growth, whereas polysaccharides reached a maximum 2 to 3 weeks later. In culture filtrates, the amount of both proteins and carbohydrates increased with the aging of tubercle bacilli; the increase of carbohydrates, however, was larger. Few quantitative differences in the yield and chemical composition of mycobacterial fractions were noted between the three tubercle bacilli.