We studied the effect of chemotherapeutic agents, doxorubicin (Dox) and 5-fluorouracil (5-FU), on normal human mammary epithelial cells (HMEC) and tumor (MCF7) cells. Dox and 5-FU exhibited nearly 20- and 2-fold more-selective toxicity for tumor cells than for normal cells, respectively. Sequential exposure (i.e., Dox followed by 5-FU, or 5-FU followed by Dox) was more toxic to both HMEC and MCF7 cells than when either agent was administered individually, indicating a synergistic interaction between the two drugs. The observed mechanism of growth inhibition by Dox was different for normal and tumor cells but was the same for 5-FU. The predominant responses of HMEC and MCF7 to Dox exposure were caspase-dependent apoptotic cell death and senescent-like terminal growth arrest, respectively. On the other hand, 5-FU exposure arrested both cell types in the G1/S-phase of cell cycle. Taken together, these results demonstrate that the different mechanisms of growth inhibition may provide an important determinant in decreasing harmful side-effects of chemotherapy.