The transport of nucleosides by human erythrocytes has been studied by measuring disappearance of 14C-labelled uridine and thymidine from incubation media; saturation kinetics and competitive inhibition of uridine uptake by formycin B were observed. Uridine and thymidine transport were nonconcentrative under incubation conditions in which cellular ATP concentrations were maintained. Efflux of uridine or thymidine from preloaded cells was induced by an inward flow of various nucleosides, but not by purine or pyrimidine bases. It is concluded that the transport of nucleosides across the plasma membrane of human erythrocytes is accomplished by a nonconcentrative, 'facilitated diffusion' mechanism of broad specificity.