increased incorporation of 5fluorodeoxyuridine into dna of human tlymphoblastic cell lines various human leukemoblastoid cell lines in logarithmic growth were incubated for 16 hr with 05 microm 3hfluorodeoxyuridine fdurd and the incorporation of 3hfdurd into dna was measured tacute lymphoblastic leukemia tall cell lines incorporated 2470 times more 3hfdurd into newly synthesized dna than cell lines from blymphoid pre ball null cell all and acute myeloid leukemia cells tall cells incorporated 193315 pmol of 3hfdurd nucleotides into dna per 107 cells the amount of 3hfdurd incorporated into dna was inversely correlated with the activity level of uracil dna glycosylase of the cells on the other hand no correlation was observed between the incorporation and the level of deoxyuridine triphosphate diphosphohydrolase of each cell line however the amount of fdurd incorporated into dna could not be correlated with the antiproliferative activity of fdurd against these cell lines