* Both suspensions of H37Rv and LR. v were adjusted to 10 per cent transmittance in the Coleman Universal Spectrophotometer. t Initially mice were infected with H37Rv, but thereafter with LRv. weight and plate counts on oleic acid-albumin agar. These results confirmed the purity of the suspensions of lung bacilliand established a basis for quantitative biochemical comparison of tween-grown and lung-grown tubercle bacilli. Biochemical methods. An index of general metabolic activity was obtained by testing the hydrogen transfer capacity of bacilli in the pres-ence of substrate. This was done by measuring in the Coleman Universal spectrophotometer the reduction of 2, 3, 5-triphenyl tetrazolium chloride after 2 hr of incubation in the presence of tween-albumin medium containing 0.5 per cent glucose (Vandiviere et al., 1952). The specific biochemical test used throughout this study was the comparative response of H37Rv and LRv to various substrates, as meas-ured by oxygen consumption using standard Warburg manometry. The gas phase was air, the temperature was 37.8 C, CO2 was absorbed by 0.2 ml of 20 per cent KOH in the center well, and the total volume of liquid in the Warburg vessels was 2.2 ml. The oxygen uptake of 5 to 6 mg dry weights of bacilli was determined over 2-or 3-hr periods and from these values the Qo, was determined. Endogenous respiration was determined on bacilli suspended in M/15 phosphate buffer, pH 6.8, which also was the final pH of the various substrates in the Warburg vessels. Comparative substrate response is expressed as per cent in-crease in the Qo, value over that of the endoge-nous respiration.