Avian and mammalian fibroblast cultures transformed by type C sarcoma viruses show a dramatic enhancement of the rate of hexose transport at the beginning of transformation which is quantitatively and qualitatively different from that seen by variation in culture conditions of nontransformed control cells. The identification of this change as being a transport alteration independent of total glucose metabolism has been shown by use of nonmetabolizable analogues, 2-deoxyglucose, 3-O-methylglucose, and L-glucose. Increased transport rates were not dependent on levels of hexokinase activity. Transport studies of 3-O-methylglucose confirmed these conclusions and further revealed an additional altered nature of hexose transport after transformation by sarcoma virus. 3-O-methylglucose was not only transported more rapidly in the transformed cells than in the parental nontransformed cells, but the sugar "infiltrated" into the transformed cells despite the inhibitory effect of cytochalasin B. This was not seen with control cells. The sarcoma cells were also able to transport L-glucose in contrast to lack of uptake by nontransformed cells. Under conditions in which cell toxicity was not a factor, 2-deoxyglucose and several other sugars present in culture media inhibited transformation by sarcoma viruses. These same sugars reduced the incidence of sarcomas produced by virus in vivo when administered daily to test animals. The transport changes also correlate well with the transformed state as found by other laboratories using temperature-sensitive mutants and revertant cell lines. Collectively these data suggest that manipulation of transport systems may prove useful for control of certain malignancies.