Antitumor activity of several 3',5'-diesters of 5-fluoro-2'-deoxyuridine (FUdR) against L1210 leukemia cells following intraperitoneal administration was examined. Esters of FUdR with aromatic acid or aliphatic acid of longer chain length were markedly active. Their activities, with respect to ILS30, were as much as 100 times that of unesterified FUdR. 3',5'-ditoluoyl FUdR also had an improved therapeutic effect: its therapeutic ratio was increased to 8.1, as against 2.0 for FUdR. On the other hand, 3',5'-diesters of FUdR with aliphatic acid of shorter chain length do not appear to be as active as FUdR. The relationship between the antitumor activity and plasma levels has also been examined. After 3',5'-diacetyl FUdR, which is one of the drug group showing low cytotoxicity, the plasma concentration rapidly decreased to an unmeasurable level 3 h after dosing. This tendency is similar to that shown in FUdR. On the other hand, with 3',5'-dipalmitoyl FUdR and 3',5'-dibenzoyl FUdR, each of which has a marked antitumor effect, plasma concentrations decreased slowly and were maintained for as long as 48 h after dosing. The results show that the cytotoxicity of diesters of FUdR is correlated with the duration of a high plasma level of FUdR.