A patient with mild hemolytic anemia and osmotically fragile, stomatocytic erythrocytes was studied. Analysis of the erythrocyte membrane proteins by SDS-PAGE revealed a partial deficiency of protein 4.2. All the other membrance proteins in the erythrocytes were normal. Enzyme activities in the erythrocytes were normal or increased. Deficieny of protein 4.2 is likely to be associated eith the hemolytic anemia and stomatocytosis in this patient.