Nafcillin sodium monohydrate (100 mg/kg; s.c.) exhibits bactericidal activity against methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA), with MICs of 0.5 μg/mL and 64.0 μg/mL for S. aureus strains Xen-29 and Xen-1, respectively, in mice.