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IN the accompanying communication, Droop gives a calculation to demonstrate that vitamin B12is present in the sea in amounts more than sufficient for the known plankton crops. It might be inferred from it that vitamin B12is not a factor which influences the amounts and types of life present in the sea. There probably are, however, several modifying circumstances of which Droop, because of the incompleteness of available information, could take no account. His calculations are based on determinations of the maximum yield of organisms obtainable per unit amount of vitamin in laboratory pure culture ; but it is the influence of the vitamin on the rate of growth, not on the final yield, which will be more relevant to marine ecology. In a natural environment a concentration of vitamin B12(or indeed of any other nutrient) sufficient to support considerable cell-division may nevertheless be a limiting concentration, if the rate of cell-division so supported is only sufficient to compensate for the losses of cells due to other factors such as predation, sinking and the like.