1. Tumour cells were starved to deplete them of ATP and transferred to 0·9mm-glycine in Ringer solutions containing 2mm-sodium cyanide and various Na+ and K+ concentrations. The uptake of glycine then usually reached a peak by about 10min. 2. When cellular [Na+] and extracellular [Na+] were each about 30m-equiv./l., the maximum amount of glycine absorbed increased between 1·2- and 3·0-fold on lowering extracellular [K+] from 128 to 10m-equiv./l. 3. When extracellular [Na+] was 150m-equiv./l., the ratio, R, of the cellular to extracellular glycine concentrations increased progressively, from near 1 to about 9, when cellular [Na+] was lowered from 120 to 40m-equiv./l. 4. When cellular [Na+] was almost constant, either at 45 or 70m-equiv./l., R fell about 14-fold when extracellular [Na+] varied from 150 to 16m-equiv./l. 5. Values of R near 0·2 were found when cellular [Na+] was about four times as large as extracellular [Na+]. 6. R fell about threefold when the cells were put with 12mm- instead of 0·9mm-glycine. 7. The results were taken to imply that, under these conditions, the spontaneous movements of both Na+ and K+ across the cell membrane, down their respective concentration gradients, served to concentrate the glycine in the tumour cells (Christensen's hypothesis).

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