Exposure of human tumor cell lines to different chemotherapeutic drugs, ionizing radiation, and differentiating agents induced morphological, enzymatic, and ploidy changes resembling replicative senescence of normal cells. Moderate doses of doxorubicin induced this senescence-like phenotype (SLP) in 11 of 14 tested cell lines derived from different types of human solid tumors, including all of the lines with wild-type p53 and half of p53-mutated cell lines. SLP induction seemed to be independent from mitotic cell death, the other major effect of drug treatment. Among cells that survived drug exposure, SLP markers distinguished those cells that became terminally growth-arrested within a small number of cell divisions from the cells that recovered and resumed proliferation. SLP induction in breast carcinoma cells treated with retinoids in vitro or in vivo was found to correlate with permanent growth inhibition under the conditions of minimal cytotoxicity, suggesting that this response may be particularly important for the antiproliferative effect of differentiating agents. The senescence-like program of terminal proliferation arrest may provide an important determinant of treatment outcome and a target for augmentation in cancer therapy.