The uptake of D-xylose by CHO-K1 cells in monolayer conditions, and by suspended L- and HeLa cells has been studied. The initial velocity is a function of exogenous xylose concentration. The estimated kinetic constants, Km and Vmax, at 37 degrees were, resp., 20 mM and 10 mmol/min for CHO cells, 12.5 mM and 2.8 mmol/min for L-cells, and at 16 degrees--3 mM and 2.6 mmol/min for HeLa cells. The transport of D-xylose was inhibited by D-glucose and phlorizin. It is concluded that D-xylose may penetrate into the cells by a passive transport (facilitated diffusion), and that D-xylose may be used as a transport analogue of D-glucose. The intracellular H2O space, determined from the equilibrium level of D-xylose for the monolayer of CHO cells was equal to 1.5 microliter on 10(6) cells.