An in-house method which utilizes 14C-thymidine as a substrate was used to assay deoxythymidine kinase in serum. The method is sensitive enough to detect normal levels of serum deoxythymidine kinase and the assay procedure also enables rapid handling of multiple samples. With a total reaction volume of 60 ul, the enzyme reaction was found to be linear with concentrations for up to 650 U/L of TK activity. On studying serum deoxythymidine kinase (s-TK) activity with incubation time, there was a proportional increase in activity with the length of incubation time. "Within-batch" precision showed a coefficient of variation (CV) of 4.7% for serum with extremely high s-TK levels and a CV of 8.8% for serum with normal s-TK levels. S-TK showed a CV of less than 16.0% in its activity when stored at -8 degrees C and at -20 degrees C. The normal reference range obtained for s-TK activity was 8.6 +/- 7.5 U/L.