Mutant mice reported to lack carbonic anhydrase isoenzyme II (CA II) have been examined here for immunocytochemical evidence of CA II and for histopathologic change. All histologic sites that immunostain for CA II in a wide range of organs in normal mice failed to show such immunoreactivity in the homozygous mutants. The CA II-deficient mice differed from controls in evidencing an age dependent medial calcification of small arteries in a number of organs. The male genital tract revealed the most extensive arterial calcinosis and males were possibly more affected in general than females. One or another Car-2n/Car-2n mouse showed changes additionally in uterus, small bowel, lymph nodes, or renal pelvis.