Concentrations of creatine kinase (CK) B subunit (CK-B) in tumor tissues and in sera of patients with various lung carcinomas were determined, together with the concentrations of neuron-specific γ-enolase (γ subunit of αγ and γγ enolases), by the use of a sensitive enzyme immunoassay method. The CK-B and γ-enolase levels were enhanced in tissues of small cell carcinoma of the lung. The average tissue contents of CK-B in small cell carcinoma (SCCL), adenocarcinoma (ADCL) and squamous cell carcinoma (ECCL) of the lung, and normal lung were 2320, 308, 163, and 372ng/mg protein, respectively. The contents of γ-enolase in those tissues were 1460, 276, 225, and 42.7ng/mg protein, respectively. Serum CK-B concentrations in healthy adults (n=100) were 0.53±0.22ng/ml and ranged from 0.25 to 1.44ng/ml, but they were significantly increased (>1.5ng/ml) in some patients with SCCL (26/42 cases, 62%), ADCL (7/36, 19%), ECCL (7/37, 19%), and large cell carcinoma of the lung (LCCL, 4/13, 31%). Serum CK-B was also enhanced in some patients with breast carcinoma and in a few cases in carcinomas of the stomach, colon and pancreas. Serum concentrations of CK-B were well correlated with those of γ-enolase in patients with SCCL (r=0.667, n=83, P<0.01) and LCCL (r=0.689, n=20, P<0.01), but poorly in patients with ADCL and ECCL. Since serum CK-B concentrations in patients with SCCL changed in parallel with the clinical course during treatment, serum CK-B may also be a useful biomarker, as well as neuron-specific γ-enolase, for monitoring the clinical course of patients with SCCL.