Transformation by Rous sarcoma virus results in a dramatic increase in the rate at which the transformed cells transport glucose across the cell membrane. The increased transport rate is a consequence of an increased number of transporters in the transformed cells. Utilizing antibody raised against the purified human erythrocyte glucose transporter, we have identified the glucose transporter as a membrane glycoprotein with a monomer Mr of approximately 41,000. The increased rate of glucose transport is dependent on the activity of pp60src, the transforming protein of Rous sarcoma virus. This protein has been shown to be a protein kinase that phosphorylates on tyrosine residues. We have examined the tyrosine phosphorylation of a major cellular protein of Mr 36,000 in cells infected with a panel of partially transforming mutants of Rous sarcoma virus. One of these mutants (CU2) increases the rate of glucose transport only slightly and does not render the infected cells fully anchorage independent or tumorigenic (although other transformation parameters are fully induced). Cells infected with this mutant display a 36,000-dalton protein that is phosphorylated to a considerably lesser extent than cells infected with wild-type virus. Analyses of this sort may help to identify the cellular targets of pp60src whose phosphorylation is necessary for the increased glucose transport rate.