A new variant of human erythrocyte carbonic anhydrase II (CA II) was discovered in a single Caucasian family during routine screening of blood samples from Melbourne, Australia. The normal and variant enzymes in the heterozygous CA II mixture, as well as a minor component of the normal enzyme, were resolved by isoelectric focusing following purification by a specific affinity matrix. Specific esterase activities of all three were very similar, but quite different Michaelis-Menten constants were noted for the minor component. No differences were noted with respect to inhibition by acetazolamide, but the minor component was more sensitive to chloride inhibition. Double diffusion analysis showed the immunological identity of the normal, variant, and minor components. Both the variant CA II and the minor component were less heat stable than the normal enzyme, but all forms showed identical rates of inactivation upon dialysis against the zinc chelator pyridine dicarboxylic acid. Amino acid analyses of the whole protein and the single difference peptide were consistent with a proline to histidine substitution in the variant. This was identified as 237 Pro leads to His by a process of elimination involving direct sequencing of tryptic and cyanogen bromide peptides. The numbering is by homology with the human CA I sequence.