There is some evidence in the literature that the pharmacokinetics of anticancer agents can be influenced by the presence of a tumor. Therefore several authors recommend pharmacokinetic studies of such drugs to be performed also in tumor-bearing animals (2, 5, 7). The aim of the present study was to evaluate the influence of different stages and routes of inoculation of leukemia P 388 in B6D2F1-hybrid mice on the tissue distribution of ambazone, a new potential antineoplastic drug. It could be emphasized that the drug levels in liver, kidneys and thymus were higher in advanced tumor-bearing than in control animals whereas in the spleen and in whole blood the opposite was true. The differences can be explained partially by changes in the erythrocyte binding of ambazone.