The effects of phenylhydrazine on intact red cells and on red cell ghost membrane proteins were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In intact red cells 1 mM phenylhydrazine induced a marked decrease in intensity of the alpha- and beta-bands of spectrin without the formation of high molecular weight materials. Phenylhydrazine was also responsible for cross-linking of hemoglobin, which is apparent by the appearance of two new broad bands on the gel. Membrane glycoproteins were unaffected. Electrophoretic patterns of cytoskeletal proteins from phenylhydrazine-treated red cells obtained on two-dimensional SDS-polyacrylamide gels and stained with Coomassie blue or fluorescently labeled with monobromobimane indicated the presence of a new band between bands 4.2 and 5 at 60-65 kilodaltons (K). An immunoelectrophoretic blotting procedure utilizing polyclonal IgG antibodies for alpha- and beta-spectrin of the red cell cytoskeletal proteins revealed that the band observed at 60-65 K in the two-dimensional SDS-PAGE studies reacted with the antibodies. The presence or absence of glucose in the incubation medium and modification of oxyhemoglobin to met- or carboxyhemoglobin in the red cells did not protect the phenylhydrazine-mediated degradation of the major cytoskeletal proteins. Metal chelators and antioxidants had no effect on membrane protein changes. Ghost red cell proteins did not undergo changes at 1 mM phenylhydrazine in the presence or absence of hemoglobin, although at 5 mM phenylhydrazine the appearance of a faint high molecular weight band was observed. These results indicate that spectrin degradation without significant polymerization can be induced by phenylhydrazine.