The 67-kDa subunit A of the vacuolar-type H⁺-ATPase carries the high affinity ATP binding site and together with the 57-kDa subunit B forms the catalytic domain. Two isoforms of subunit A, VA68 and HO68, were cloned from an osteoclastoma cDNA library. We have analyzed their respective expression patterns in different tissues by RNAse A protection and in situ hybridization. The HO68 isoform was found to be present only in the tumor originally used to construct the cDNA library, whereas the ubiquitous VA68 RNA isoform was detected in large osteoclastic cells, as well as in brain and kidney, by RNAse protection assay. Furthermore, we localised the strong signal observed in osteoclastoma RNA to the large osteoclastic cells by in situ hybridisation. These findings suggest that the subunit A highly expressed in human osteoclasts is the ubiquitous isoform, VA68.