We have evaluated a new sensitive immunometric assay for the determination of creatine kinase (CK; EC 2.7.3.2) MB isoenzyme (NovoClone CK-MB), involving an enzyme label and two monoclonal antibodies directed against the B subunit and the M subunit, respectively. The anti-CK-B antibodies are bound to the solid phase. The assay was modified to be extremely sensitive and thus to measure the concentration range below and close to the cutoff value used for the diagnosis of myocardial infarction. A reference interval of 0-6 micrograms/L was found for 315 outpatients without myocardial diseases (132 men and 183 women); the overall median of the log-gaussian distribution was 1.91 micrograms/L (2.03 and 1.79 micrograms/L for men and women, respectively). Total and within-assay imprecision (CV) was less than 6% at the upper reference limit. The detection limit was 0.1 microgram/L. The assay provides a favorable signal-to-noise ratio: the calibrators 0.0, 2.0, and 30.0 micrograms/L give absorbances at 492 nm of 0.040, 0.140, and 1.600 A. respectively. We conclude that the assay provides biochemical identification of individuals with myocardial damage but without myocardial infarction.