Anti-microbial cecropins are humoral immune peptides originally found in insects. They possess a particular function of membrane permeabilization on both gram-positive and gram-negative bacteria. Yet, they are not capable of lysing eucaryotic cells. In this experiment, we confirmed that cecropin A (CA) is a special agent able to lyse leukemia cells while exerting little toxic effect on normal lymphocytes, based on the IC50 measurements. The therapeutic index of CA ranged from 6 to 17. In contrast, chemical anti-cancer agents (ACAs) are non-specific drugs capable of damaging both cancer and normal cells. Due to the functional differences between CA and ACAs, the experiments on the combined effects of CA and ACAs were conducted in vitro. Under certain conditions, synergistic effects were found with CA/S-FU and CA/Ara-C combinations. The results suggest the need for further study into the possible application of combining anti-cancer agents with antibacterial peptides for cancer treatment.