The dose response curves of doxorubicin for hematopoietic rat bone marrow cells were investigated and compared with the dose-response curves of doxorubicin for leukemia cells from bone marrow and spleen of rats inoculated with an acute myelocytic leukemia (BNML). Various assays were used to determine the cytotoxicity of doxorubicin. It was found that the inhibition of DNA synthesis by doxorubicin compared well with results obtained with in vivo assays for the determination of clonogenic hematopoietic (CFU-S) and leukemic (LCFU-S) cells. It was found that doxorubicin at concentrations ranging from 0.1--1.0 mu g . 10(-7) cells inhibits DNA synthesis of leukemic cells to 60% and that of hematopoietic cells to 90%. Higher doxorubicin concentrations further inhibit DNA synthesis of only hematopoietic cells. These results were confirmed with clonogenic assays. Pretreatment with the S phase-specific drug arabinoside cytosine (ara-C) increased the efficacy of doxorubicin in vitro significantly. In view of the doxorubicin concentrations in bone marrow obtained in vivo (Greater Than or Equal To 1 mu g . 10(-7) cells), it is concluded that dosage reduction may reduce toxicity with no concomitant decrease of antileukemic activity of doxorubicin.