Monocytes from blood and from spleen have been cultivated in fluid medium in Carrel flasks for over 2 months. Diluted serum supplied all the essential nutritive substances. Cultivation in fluid was made possible by adjusting the initial pH of the fluid to 7.4, and not allowing it to fall below 7.0 or 6.8. The cells remained in good condition when the pH was adjusted with either lactic acid, hydrochloric acid, or carbon dioxide. Adjustment with carbon dioxide was found to be more convenient and also more practical, since it does not destroy the buffer action of the medium. After 2 months of cultivation, the monocytes were in excellent condition and still proliferated actively. They gave every indication that indefinite multiplication could be maintained under the conditions of these experiments. It is hoped that this method of cultivation, with some modifications, will prove useful in studying the metabolism of these cells.

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