The differential tissue uptake and blood clearance of [2-14C]-5-ethyl-1-(2'-deoxy-beta-D-ribofuranosyl)uracil [2-14C]-5-EtUdR) in BDF1 mice bearing Lewis Lung carcinomas has been investigated. The kidney, gall bladder with contents and GIT exhibited the greatest concentration of radioactivity relative to the tumor up to 30 min after i.v. injection of [2-14C]-5-EtUdR. Blood, spleen, liver and muscle exhibited a lower concentration of radioactivity than the former group. Radioactivity levels in the stomach and skin were similar to that of the tumor. The long bone had the lowest radioactivity level of all the tissues examined. During the 1-3 h period following injection, the tumor contained more radioactivity than most other tissues with the exception of the gall bladder and contents, GIT and kidney. Packed erythrocytes contained 62-78% of the radioactivity in the whole blood. Clearance of injected radioactivity from whole blood followed two-compartment kinetics. Elimination was mainly renal, although some biliary excretion of injected radioactivity was also observed.