Distamycin nitrogen mustard derivatives with different substituents at the amidino moiety located at the C-terminal of the peptide were synthesized. The in vitro antitumor activity was determined against human chronic leukemia K562 cells. Compound 3, bearing a terminal ethylamido group, had the best antitumor activity with an IC50 value of 0.72 μM. Compound 5, bearing a terminal dimethylamino group, had an IC50 value of 2.0 μM. This result suggests that a terminal positive charge is not essential for optimal antitumor activity.