Brevinin-1BYa (FLPILASLAAKFGPKLFCLVTKKC), first isolated from skin secretions of the frog Rana boylii, displays broad-spectrum antimicrobial activity and potent haemolytic activity. This study investigates the effects on conformation and biological activity of replacement of the intramolecular disulphide bridge in the peptide by a non-reducible dicarba bond. Dicarba-brevinin-1BYa was prepared by microwave irradiation of [Agl(18),Agl(24)]-brevinin-1BYa (Agl = allylglycine) in the presence of a second generation Grubbs catalyst. Circular dichroism spectroscopy in 50 trifluoroethanol-water indicated that the degree of alpha-helicity of the dicarba derivative (22 ) was less than that of brevinin-1BYa (27 ) but comparable to that of the acyclic derivative [Ser(18),Ser(24)]-brevinin-1BYa (23 ). Dicarba-brevinin-1BYa showed a two-fold increase in potency against reference strains of Escherichia coli, Staphylococcus aureus, and Candida albicans compared with the native peptide and displayed potent bactericidal activity against clinical isolates of methicillin-resistant S. aureus (MRSA) and multidrug-resistant Acinetobacter baumannii (MIC in the range 1-8 mu M). Compared with brevinin-1BYa and [Ser(18),Ser(24)]-brevinin-1BYa, the dicarba derivative was associated with increased cytotoxicity against human erythrocytes (2.5-fold), MDA-MB-231 breast carcinoma cells (1.3-fold) and HepG2 hepatoma-derived cells (1.5-fold).