Various human leukemoblastoid cell lines in logarithmic growth were incubated for 16 hr with 0.5 microM [3H]fluorodeoxyuridine (FdUrd) and the incorporation of [3H]FdUrd into DNA was measured. T-acute lymphoblastic leukemia (T-ALL) cell lines incorporated 2.4-7.0 times more [3H]FdUrd into newly synthesized DNA than cell lines from B-lymphoid, pre B-ALL, null cell ALL and acute myeloid leukemia cells. T-ALL cells incorporated 1.93-3.15 pmol of [3H]FdUrd nucleotides into DNA per 10(7) cells. The amount of [3H]FdUrd 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.