A viable anaemic mouse mutant arose in the stock of a mouse fancier and has been characterized haematologically and genetically. Anaemic animals were less viable than normal animals (especially from 0 to 2 weeks of age) and had lower haemoglobin levels, percentage packed-cell volumes, higher red cell counts and lower mean cell volume than normal animals. Peripheral blood films showed a wide range of abnormal cells and extreme microcytosis. Linkage studies showed the mutant to be linked to the chromosome 15 markers Na Ca and bt; recombination with Ca was 1·37 ± 0·68 % for females and 10·5 ± 7·41 % for males. This position is similar to the microcytic anaemia, mk, mutant, and crosses between the viable anaemia mutation and mk/mk homozygotes showed the two to be allelic. Viable anaemia is therefore a second allele at the mk locus mkvan; new data give its position on chromosome 15.