We have developed a rapid sampling technique for animal cells in suspension for the purpose of measuring membrane transport in lymphocytes. The method involves rapid centrifugation of cells through a layer of silicone oil into perchloric acid after incubation periods as short as 4 s. Using this method we have described the uptake of thymidine and the uptake and transport systems of adenosine by murine bulk nonadherent spleen cells. The two uptake systems are markedly different. Adenosine was shown to be taken up by classical carrier-mediated diffusion, while thymidine was not. In addition we have explored the metabolism of the two nucleosides under the conditions we employed for measuring transport or uptake; Both nucleosides are phosphorylated extensively. We also investigated the uptake and metabolism of thymidine over a 2-h interval the standard time used to measure DNA synthesis in lymphocytes. Unless cells were separated from medium by centrifugation through oil before TCA addition, the TCA precipitable counts exceeded the total radioactive uptake. Hence the standard method for measuring thymidine utilization yields estimates under these conditions which can be as much as 100% too high.

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