The incorporation of C14-formate into thymine is described as a method for measuring desoxyribonucleic acid synthesis by bone marrow cells in vitro. The method is satisfactorily reproducible and permits measurement of DNA synthesis over short periods of time with small amounts of bone marrow.

Normal serum was found to be essential for rapid rates of DNA synthesis and could not be replaced by supplemented media. The rate of DNA synthesis was almost constant during the first 4-6 hours of incubation, but fell off rapidly thereafter. The ability to promote DNA synthesis was found to differ from one normal serum to another. The optimal pH for DNA synthesis was approximately 7.8.

Because DNA synthesis is vitally concerned with cell duplication, measurement of the rate of its synthesis should be a valuable tool for measuring viability of those marrow cells young enough to proliferate.

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