The incorporation of (1-beta-d-arabinofuranosylcytosine (ara-C) into the DNA of leukemic cells is highly correlated with cytotoxicity in vitro. However, the measurement of ara-C incorporation into leukemic cell DNA in vivo during ara-C therapy has been limited by the lack of a suitably sensitive method. A quantitative assay procedure has therefore been developed to determine incorporation of unlabeled ara-C into DNA. This method involves DNA isolation from patient myeloblasts, enzymatic digestion of the DNA, high pressure liquid chromatography separation of the nucleosides, and determination of ara-C in the eluate fractions by radioimmunoassay. Using this approach, incorporation of unlabeled ara-C into DNA of HL-60 cells is log linear over concentrations of 1 to 100 microM ara-C. Furthermore, the extent of ara-C incorporation into DNA as determined by this method correlates significantly with measurements of [3H]ara-C (DNA) formation under similar conditions. This approach has also been applied to clinical samples. Myeloblasts from 6 patients receiving high-dose continuous-infusion ara-C therapy incorporated 0.00-0.36 pmol ara-C/microgram DNA during 24 h of therapy. These findings thus suggest that this method can be used to monitor the in vivo incorporation of ara-C into leukemic cell DNA.