The effects of some mitomycin antibiotics on the chloroplast system of Euglena gracilis were studied. Only those derivatives which contained an alkyl group on the aziridine nitrogen were effective bleaching agents. Thus, only N-methyl-mitomycin, porfiromycin, and mitomycin B caused a highly significant loss of chloroplasts. This sensitivity of the Euglena chloroplast to small structural differences in the active centers of antibiotics demonstrates the usefulness of this organism in the study of relationships between biological activity and chemical structure of antibiotics.