The stability of human creatine kinase isoenzymes was investigated under different storage conditions using specific two-site monoclonal antibody assays. In Tris-HCl buffer pH 7·5 or barbitone buffer pH 8·1 containing 5 g/L bovine serum albumin, the isoenzymes appeared to be stable for up to 3 weeks at 4°C but suffered a partial subunit dissociation and random reassociation after freeze–thawing; this dissociation was more pronounced as a result of freezing at −20°C rather than at −70°C. In contrast, creatine kinase isoenzymes stored in serum were stable at both 4°C and following freeze-thawing. High levels of heart type creatine kinase in serum showed only minor subunit hybridisation even after 12 h at room temperature. We conclude that in practical clinical situations, subunit hybridisation in serum samples is negligible. We recommend however, that isoenzyme standards for use in either two-site assays or radioimmunoassays should be stored frozen in normal serum from which endogenous creatine kinase isoenzymes have been previously removed.