SUMMARY: In cultures of the flagellate the rate of growth is determined by the concentration of cyanocobalamin up to a limit at which a maximum growth rate is achieved. In organisms grown exponentially with different rate-limiting concentrations of cyanocobalamin, the amounts of protein and nucleic acids were approximately proportional to the rates at which the organisms had grown. With increasing growth rate the concentrations of riboflavin, nicotinic acid, pantothenic acid and chlorophylls increased. Carbohydrate showed little change, whereas fat and ‘free’ amino-nitrogen diminished in concentration as the growth rate increased. In cultures of grown with limiting or near-limiting concentrations of cyanocobalamin and with increasing concentrations of chloram-phenicol, the rates of division declined asymptotically to a low value corresponding with 500 μg. chloramphenicol/ml. culture medium. Inhibition of growth caused by the antibiotic was annulled by increasing the concentration of cyanocobalamin in the culture medium.


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