We report two patients with hereditary spherocytosis associated with band 4.2 protein deficiency from a Japanese family. The defect of band 4.2 protein was confirmed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) not only in freshly prepared white ghosts but also in washed whole erythrocytes. The finding was quite reproducible and was also recognized postsplenectomy. The interaction of ankyrin with band 3 in the patients' ghosts was stable both at low ionic strength and at acidic pH. Our results suggested that band 4.2 protein might not be essential for the structural stability of band 3-ankyrin interaction. On the other hand, membrane protein phosphorylation studies revealed an increased phosphorylation of spectrin/ankyrin, band 3 and band 4.1 in the patients' erythrocytes as compared with normal cells. The finding might be related to a dysregulation of protein phosphorylation which could result in membrane instability in affected cells. Band 4.2 deficiency is an inherited disorder in association with hereditary haemolytic anaemias and seems to be relatively prevalent in the Japanese population.