A dog with persistent elliptocytosis was studied. The dog had membrane protein band 4.1 deficiency, microcytosis, shortened erythrocyte lifespan, increased osmotic sensitivity, and a mild glutathione deficiency. Erythrocyte deformability and membrane stability were adversely effected. The dog's parents had decreased band 4.1, decreased stability, and some elliptocytosis. This disorder in dogs closely resembles human patients with band 4.1 deficiency and should provide a valuable animal model to study the erythrocyte membrane cytoskeleton.