Lactivicin is moderately active against a wide range of Gram-negative bacteria and highly active against Gram-positive bacteria. It shows various biological activities commonly observed with β-lactam antibiotics, such as higher activity against β-lactam hypersensitive mutants than against their parents, sensitivity to β-lactamases, inhibitory activity against β-lactamases and ability to induce β-lactamase activity. The primary lethal target of lactivicin in Escherichia coli is highly likely to be penicillin-binding protein (PBP) 1; lactivicin strongly lysed E. coli cells with induction of spheroplasts at its MIC, and showed high affinity for PBPs 1A and IB. At concentrations above × 5 MIC, however, lactivicin dominantly exhibited secondary antibacterial action possibly owing to inhibition of crucial SH proteins engaged in the fundamental membrane functions. In contrast, against Bacillus subtilis, lactivicin showed the typical β-lactam action under a wide range of concentrations. It showed high affinity for PBPs 1, 2 and 4, the possible lethal targets of β-lactam antibiotics in this organism. In conclusion, lactivicin is the first non-β-lactam antibiotic showing β-lactam action through binding to PBPs.