The antimicrobial activity of eight cationic, two neutral and three anionic liposome compositions containing meropenem and gentamicin was tested in vitro in broth and serum medium. The cationic formulations showed better antibacterial efficacy against both Gram-positive and Gram-negative bacteria than the anionic and neutral ones, regardless of the encapsulated drug. The most effective formulations were the cationic PC/DOPE/DOTAP 3:4:3 and PC/Chol/DOTAP 3:4:3, as the MICs with meropenem were 2 to 4 times lower than those of the free drug.