Anthracycline antitumor drugs such as doxorubicin (DOX) and aclacinomycin (ACM) represent potent candidates for the induction of differentiation of leukemic cells. Human multipotent K562 cells were induced by DOX and ACM to differentiate towards the erythroid lineage. After 3 days of culture, DOX-induced differentiation was dose-related whereas ACM did not require total cell growth arrest to induce its optimum effect, indicating that both drugs act differently on the coupling of growth and differentiation. Simultaneous exposure to ACM and DOX and sequential exposure to ACM (30 min) first, followed immediately by DOX did not improve erythroid differentiation. However, it led to either a synergistic or a subadditive inhibition of cell growth. In contrast, DOX (30 min) first, followed by ACM, produced in a narrow range of concentrations (DOX 1000 nM/ACM 1.85 nM, 3.75 nM or 7.5 nM), a synergistic induction of differentiation. Thus, DOX 1000 nM/ACM 3. 75 nM resulted in 81% of differentiated cells compared to 63% for ACM 15 nM and 43% for DOX 30 nM when these were used alone (at their concentration inducing optimum differentiation). In conclusion, these data emphasize the importance of schedules for the combination of chemotherapeutic drugs acting as differentiation inducers.