During recent years, a resurgence of interest in the macrolides had led to the discovery of new derivatives of erythromycin with improved antibacterial activity and pharmacokinetic properties. In this study the in-vitro and in-vivo antistaphylococcal activity of S-5556, a 16-mciubered macrolide, was evaluated. In vitro, S-5556 was slightly less active than erythromycin against methicillin-susceptible Staphylococcus aureus. In contrast, it had superior activity for methicillin-rcsistant S. aureus (MRSA); several of these strains with inducible resistance to the macrolides-tincosamides-streptogramins group were susceptible to S-5556 whereas erythromycin was inactive. The combination of S-5556 with oxacillin was synergic for most MRSA strains tested. In vivo, a single prophylactic dose of S-5556 prevented 75%-100% of the cases of acute staphylococcal subcutaneous foreign body infection in a guinea pig-model. In a rat-model of chronic implant infection due to a methiciUin-and erythromycin-resistant S. aureus strain, S-5556 significantly decreased the bacteria] concentration around the foreign material, however resistant mutants emerged.