5-Ethyl-2'-deoxyuridine (5EtdUrd) is a biologically active thymidine analogue. The cytotoxicity of 5EtdUrd was investigated with seven established human leukemia cell lines as well as with human peripheral blood PHA-stimulated lymphocytes. All types of leukemia cells were susceptible to the toxicity of 5EtdUrd as assayed with a [U-14C]-L-leucine incorporation system developed for this study. A 50% inhibition of leucine incorporation in 3-day cultures was induced by 1.3-3.8 microM 5EtdUrd with leukemic cells, but the concentration required to induce similar inhibition with PHA-stimulated lymphocytes was approximately was approximately 100-fold. The toxicity of 5EtdUrd seemed to require active DNA synthesis, since the inhibition of leucine incorporation became obvious only after the first 24 hours of culture. The DNA incorporation studies were based on a new isotopically labeled 5EtdUrd derivative, [2-14C]5EtdUrd, synthesized for this study in our laboratory. It was demonstrated for the first time that most of the radioactivity derived from [2-14C]5EtdUrd in DNA was in 5-ethyluracil. 5EtdUrd has a powerful antileukemic potency in vitro. Its effects against human leukemia in vivo remain to be tested.