3-O-methylglucose (3-OMeG) is a nonmetabolizable glucose analog and is, therefore, suitable for transport studies. 3-OMeG and glucose compete for entry into normal and Rous sarcoma virus (RSV)-transformed chick-embryo fibroblasts. Therefore, 3-OMeG can be used to study the transport of glucose in these cells. Chickembryo fibroblasts infected and transformed by RSV take up 3-OMeG at a faster rate than uninfected cells when both cell types are growing at the same rate. The rate of efflux of 3-OMeG also increases after transformation. When the uptake and the efflux reach a steady state, the intracellular concentration of 3-OMeG is equal to the concentration in the medium. This finding indicates that glucose is transported across the plasma membrane by facilitated diffusion. The V(max) of the transport system for 3-OMeG increases after transformation, while the affinity or K(m) of the system remains unchanged. We conclude that viral transformation causes a change in the plasma membrane of the infected cells by increasing either the number of molecules or the mobility of the glucose carrier.