To clarify the clinical significance of serum deoxythymidine kinase in various hematological disorders, basic examination was performed of a newly developed radioenzyme assay kit for TK (Prolifigen TK kit "Daiichi") using 125I-iodo-deoxyuridine as a substrate. The assessment of the standard curve, and the results of recovery test, dilution test, and precision and reproducibility were satisfactory. Among several anticoagulant agents tested, heparin was the best one showing no influence on the measurement of TK level. Based on these results, serum TK level was measured in 140 healthy adults as a control group. There was no difference by age and sex. The serum TK level of normal control was 3.1 +/- 1.2 U/l, with an upper limit of 5.5 U/l.