Sterically stabilized liposomes are able to localize selectively at sites of infection, potentially permitting targeted drug delivery. Up to now, the majority of studies investigating therapeutic efficacy of liposomes have been conducted in animals with an intact host defense infected with high antibiotic-susceptible bacteria. In the present study, the therapeutic efficacy of gentamicin encapsulated in sterically stabilized liposomes, alone or in combination with the free drug was studied in rats with intact host defense as well as leukopenic rats. Rats were inoculated with a high gentamicin-susceptible or low-gentamicin susceptible Klebsiella pneumoniae in the left lung, resulting in an acute unilateral pneumonia. Survival rates demonstrate the valuable therapeutic properties of the liposome-encapsulated drug in these clinically relevant animal models.