The antitumor activity of GP-7, a new spin-labeled epipodophyllotoxin, was studied by liquid scintillation spectrometry. There were many similarities between GP-7 and etoposide. Both GP-7 and etoposide inhibited the incorporation of [3H]TdR, [3H]UR, and [3H]Leu into DNA, RNA, and protein synthesis in leukemia 7712 cells. The inhibition correlated with drug concentration and duration. IC50 of GP-7 and etoposide on DNA synthesis at 24 h were 0.21 and 0.37 micrograms.ml-1, respectively. The inhibition of GP-7 or etoposide on DNA synthesis retained even after the drug were washed out for 3 h. GP-7 and etoposide caused DNA single-strand breaks, with a well concentration-response relationship. These data suggest that the inhibition of DNA synthesis by GP-7 or etoposide is likely due to the damage of DNA template and breaking of single-strand DNA.