Adenylate kinase activity (AK) originating from erythrocytes, present in hemolyzed serum behaves like creatine kinase MM isoenzyme (CK-MM) in some CK electrophoresis assays that employ, in their visualization reagent kits, adenosine monophosphate (AMP) as the sole inhibitor ofAK, rather than a combination of AMP and a more potent inhibitor of erythrocyte AK, diadenosine pentaphosphate (ApSA), to inhibit all contaminating-AK activities in serum and quantify only the CK isoenzyme activities in serum following electrophoretic fractionation on agarose gel.This can spuriously overestimate the CK-MM fraction and thereby result in underestimation of Key words: CK isoenzyme fractionation, adenylate kinase inhibition, erythrocyte AK inhibitor, diadenosine pentaphosphate, CK electrophoresis CK-MM or CK-BB isoenzymes if present. A hemolyzed serum sample obtained from an elderly patient was erroneously reported as containing low CK-MB due to such overestimation of CK-MM fraction in the sample. Supplementing the AMP already present in the visualization reagent formulation, used to estimate CK isoenzyme concentration in serum, withAp5Acan eliminate or effectively minimize AK interference, especially that caused by hemolysis, and thereby prevent reporting false-negative CK-MB result obtained with CK isoenzyme electrophoresis assays. o 1994 Wiley-Liss, Inc.