When characterized hematologically, anemic animals were less viable than normal, and had lower hemoglobin levels, lower percentage packed cell volumes, higher red cell counts, and lower mean cell volume than normal mice. A wide range of abnormal cells and extreme microcytosis was found in peripheral blood films. Linkage studies localized the mutation to Chr 15, at or near the Slc11a2^mk locus, and complementation studies showed that this allele was another allele at that locus (J:9161).