Chlamydocin is a powerfulin vitro antitumoral agent, quickly inactivatedin vivo. A series of cyclic tetrapeptides related to chlamydocin or HC toxin and bearing a bioactive alkylating group on anε-amino-lysyl function have been examined for their antitumoral activity on L1210 and P388 murine leukemia cell lines. One analog was found to be potent at inhibiting L1210 cell proliferation and had a higher therapeutic index than the reference compound bis-β-chloroethylnitrosourea on thein vivo P388-induced leukemia model.