We have developed a quantitative immunoassisted enzyme assay to screen for creatine kinase isoenzymes BB (CK-BB) and mitochondrial (CK-m) using commercial antibodies and reagents. Presence of CK-m activity was subsequently confirmed by electrophoretic separation of samples with elevated values. A prospective clinical trial was undertaken in 117 subjects: Normal (30), cirrhosis patients (30), myocardial infarction patients (30), and untreated oncology patients with metastatic malignancy (27). In 12 patients with malignancy CK-m activities were elevated; all had adenocarcinomas. No significant activity was detected in patients with other malignancies. CK-m positive tumour patients had a significantly higher mortality rate, and in two instances death was preceded by a sudden rise in CK-m activity. We suggest CK-m is a marker of adenocarcinoma and its presence in serum signifies increased mortality.