A separation between mecA⁺ strains of Staphylococcus aureus and strains lacking mecA was achieved by the disk diffusion assay and the agar dilution method, utilizing disks containing 5 µg of oxacillin and inocula of approximately 5 × 10⁵ CFU/spot, respectively, provided that agar with 0 to 0.5% NaCl and incubation at 30°C were employed. The 5-µg oxacillin disks clearly discriminated between borderline methicillin-susceptible and mecA⁺ strains. The oxacillin MICs were more affected by the inoculum density and salt concentration than were the methicillin MICs, and oxacillin MICs of 4 to 16 µg/ml were obtained for strains lacking mecA. Significantly higher levels of -lactamase production and reduced oxacillin susceptibilities were recorded for strains lacking mecA, in particular strains of phage group V, when agar with 2% NaCl was used than when agar with 0 to 0.5% NaCl was employed. The results indicate that the borderline methicillin-susceptible phenotype is a salt-dependent in vitro phenomenon of questionable clinical relevance.