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The possibility that j3 is affected by working pressure might be inferred from these results but it is felt that a conclusion is only justified when it is supported by direct measurements of the amount of oxygen abstracted from the air. This problem is now being studied using an oxygen recorder which measures paramagnetism. ConclusionsThese results show that at a sulphite oxidation rate of 200-250 millimoles/l./h., whichwe regard as a practical maximum for this size of vessel when working at atmospheric pressure, it is only possible for bacteria to grow exponentially to a concentration of about 3 g./l. This is of particular importance if it is proposed to grow them in continuous culture because a limit is automatically set to rate of output. Alternatively, in batch culture it means that if high concentrations are desired, the concluding stage of growth must be carried out at a restricted growth rate. If A . cloacae can be regarded as a typical bacterium this fact poses the question whether the now traditional agitated deep culture is the ideal method for mass production of. bacteria, and suggests the need for continuing investigation of means of improving oxygen supply in culture vessels.