Hereditary spherocytosis is characterized by a reduced spectrin content of the erythrocytes. However, the underlying primary defect remains unclear in the majority of cases. Genetic studies have revealed a linkage to the gene for ankyrin in some families. By means of ELISA we measured the ankyrin, spectrin, and band-3 contents in erythrocytes of 45 patients with typical spherocytosis. They were classified as having mild or moderate spherocytosis, according to clinical severity. Sixteen patients with mild spherocytosis showed slight reductions of ankyrin and spectrin contents. In contrast, 29 patients with moderate spherocytosis exhibited a clear reduction of both ankyrin and spectrin to about 60% of normal. Band 3 and lipid phosphorus, as measures for membrane surface area, were only slightly reduced to 85%. Our results, together with the molecular genetic data indicating the linkage between spherocytosis and the gene for ankyrin, suggest an ankyrin defect or deficiency as the primary lesion in most cases of spherocytosis.