Malformin C is a natural fungus-derived bicyclic pentapeptide that has antibacterial properties, particularly against species of Bacillus. Malformin C potently blocks the ability of bleomycin to induce G2 arrest in human T-cell leukemia-derived Jurkat cells (IC50 = 0.9 nM). It less potently abrogates colchicine-induced M phase arrest in Jurkat cells (IC50 = 24 nM). Malformin C inhibits cell growth dose-dependently in Colon 38 and HCT 115 cancer cells (IC50s = 0.27 and 0.18 μM, respectively) but has a low therapeutic index against cancer xenografts when tested in mice.