In previous in-vivo studies, we demonstrated that liposomal entrapment of tobramycin resulted in an increased availability of the antibiotic in the lungs without increasing bactericidal efficacy (Omri et al. 1994). With the aim of developing liposomal formulations allowing more efficient liposome-bacteria interactions, we studied the influence of lipid composition on both drug release and pulmonary retention of encapsulated tobramycin. The phase transition temperatures of nine liposome-tobramycin formulations consisting of two synthetic phospholipids (distearoyl phosphatidylcholine (DPSC) or dipalmitoyl phosphatidylcholine (DPPC) with dimyristoyl phosphatidyl-glycerol (DMPG) or dimyristoyl phosphatidylcholine (DMPC)) were determined by differential scanning calorimetry. Liposomes, varying in terms of membrane fluidity and charge were submitted to in-vitro and in-vivo kinetic studies while retention and release of tobramycin were measured by high-performance liquid chromatography (HPLC). Five less fluid liposome formulations showed absence or very low tobramycin release in in-vitro tests and long term pulmonary retention of tobramycin. Four fluid liposome formulations showed in in vitro tests modulated tobramycin release while pulmonary retention of tobramycin was dependent of the presence of charged phospholipids. Administration of charged fluid liposomes in mice showed a low level of tobramycin in the kidneys; non-charged fluid liposomes exhibited a relatively high level of tobramycin retention in the kidneys.