The proline‐rich designer antibacterial peptide dimer A3‐APO is currently under preclinical development for the treatment of systemic infections caused by antibiotic‐resistant Gram‐negative bacteria. The peptide showed remarkable stability in 25% mouse serum in vitro, exhibiting a half‐life of ∼100 min as documented by reversed‐phase chromatography. Indeed, after a 30‐min incubation period in undiluted mouse serum ex vivo, mass spectrometry failed to identify any degradation product. The peptide was still a major peak in full blood ex vivo, however, with degradation products present corresponding to amino‐terminal cleavage. When injected into mice intravenously, very little, if any unmodified peptide could be detected after 30 min. Nevertheless, the major early metabolite, a full single‐chain fragment, was detectable until 90 min, and this fragment exhibited equal or slightly better activity in the broth microdilution antimicrobial assay against a panel of resistant Enterobactericeae strains. The Chex1‐Arg20 metabolite, when administered three times at 20 mg/kg to mice infected with a sublethal dose (over LD₅₀) of an extended spectrum β‐lactamase‐producing Escherichia coli strain, completely sterilized the mouse blood, similar to imipenem added at a higher dose. The longer and presumably more immunogenic prodrug A3‐APO, injected subcutaneously twice over a 3‐wk period, did not induce any antibody production, indicating the suitability of this peptide or its active metabolite for clinical development.