A new mutation causing spherocytic, hemolytic anemia has been discovered in the house mouse. It is inherited as a single autosomal recessive gene, allelic with both sph and ha, which, in turn, were shown to be allelic with each other. A revised nomenclature for the three apparent alleles is proposed: sph (formerly sph), sphha (formerly ha), and sph2Bc (the new mutation). Like the other murine hemolytic anemias, sph2Bc involves a defect in the red blood cell membrane protein, spectrin.