DNA polymerase .alpha. from the leukemic cells of acute lymphoblastic leukemia (ALL) was more resistant to the inhibition by 1-.beta.-D-arabinofuranosyl CTP than that from acute myeloblastic leukemia (AML). Apparent Ki values for 1-.beta.-D-arabinofuranosyl CTP of DNA polymerase .alpha. from 8 patients with ALL [26.7 .+-. 7.1 (SD) .mu.M) were 5 times higher than those from 9 patients with AML [5.2 .+-. 1.4 .mu.M]. Apparent Km values for a normal substrate deoxy CTP of DNA polymerase .alpha. preparations from either AML and ALL were almost identical (9.4-10.9 .mu.M). Likewise, apparent Ki values for another arabinoside analog, 9-.beta.-D-arabinofuranosyl ATP, of DNA polymerase .alpha. from blasts of 7 patients with ALL (16.9 .+-. 6.9 .mu.M) were significantly higher than those from 4 patients with AML (3.8 .+-. 0.5 .mu.M). Apparently, DNA polymerase .alpha. from ALL blast cells has a decreased affinity to the arabinoside analogs of deoxynucleotide triphosphate. The sensitivity of DNA polymerase .alpha. of blast cells to 1-.beta.-D-arabinofuranosyl CTP may be one of the determinants of the clinical response to 1-.beta.-D-arabinofuranosylcytosine [cancer chemotherapy] treatment.