Hereditary spherocytosis (HS) is a congenital hemolytic anemia which is characterized by spherocytes in peripheral blood and increased osmotic fragility test. The disease is caused by defects in red cell membrane cytoskeleton. In this study, we investigated erythrocyte membrane protein defects in 50 Turkish HS patients and 42 controls. We used sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to identify the protein defects causing HS. The patients were from 27 families (39 kindred and 11 unrelated patients). They were aged between 6 months and 53 years and the mean age was 18.75 (+/- 14.70) years. Protein deficiencies related to HS were demonstrated in 42% of study group. There was not any statistically significant relation between the protein deficiency and hemoglobin levels. Isolated or combined spectrin deficiency was the most common protein abnormality among our patients. Spectrin deficiency was detected in 22% of cases (11/50), ankyrin deficiency in 8% (4/50), protein 4.2 deficiency in 8% (4/50), combined spectrin and protein 4.2 deficiency in 2% (1/50), combined spectrin and ankyrin deficiency in 2% (1/50). Fifty-eight percent of cases (29/50) showed normal protein contents.