[3H]Cytochalasin B was found to be a natural photoaffinity ligand which at low concentrations could be photoincorporated into specific proteins of the human erythrocyte plasma membrane associated with high affinity binding sites of this molecule. In the presence of L-glucose, the polypeptide region referred to as zone 4.5 on sodium dodecyl sulfate-polyacrylamide gel electrophoretograms was photolabeled with [3H]cytochalasin B. At least two major peaks could be resolved in this region, one at approximately 50,000 daltons and the other at approximately 47,000 daltons. In addition, this incorporation was partially blocked when D-glucose was substituted for L-glucose in these experiments. Incorporation was time-dependent upon photolysis, did not occur in the absence of light, and was not reversible under the harsh conditions of gel electrophoresis. This suggests a covalent incorporation of this ligand into polypeptides previously identified as being associated with monosaccharide transport in these cells. In addition, soluble G-actin from muscle was labeled, but at a level 100-fold lower than transporter polypeptides. These experiments suggest that this technique may prove useful in identifying monosaccharide transporter proteins in other cell systems as well as other types of cellular proteins associated with high affinity cytochalasin binding.