OBJECTIVES: The bla regulatory system is critical in the regulation of mecA expression particularly when staphylococci lack the mec regulator. We sought to evaluate the effect of bla regulators on the cryptic methicillin-resistant phenotype and resistance conversion under β-lactam exposure in oxacillin-susceptible or oxacillin-resistant mecA-positive staphylococcal isolates. METHODS: Methicillin-resistant staphylococci isolates, NW19 and DY39, and their mutants, were used in this study. Both NW19 and DY39 carried intact mecA, a truncated mecR1 gene and a single copy of the bla regulatory system. Oxacillin MICs were determined using the agar dilution method. Increased and reduced expression of bla regulators was achieved by overexpression and antisense RNA, respectively. Expression of mecA, blaR1 and blaI was quantified in the presence or absence of oxacillin. RESULTS: NW19 had high expression of blaR1-blaI, low expression of mecA and was oxacillin susceptible, while DY39 expressed a low level of blaR1-blaI, expressed a high level of mecA and was oxacillin resistant. Increased expression of blaR1-blaI in DY39-RI led to an oxacillin-susceptible phenotype. Overexpressing blaR1 in DY39-R did not result in any phenotypic change. Under serial exposure of oxacillin, NW19, DY39-RI and DY39-R, with higher expression of blaR1, converted to be highly resistant at a faster speed compared with DY39 and NW19-KD, which had lower expression of blaR1. CONCLUSIONS: The expression level of BlaI was mainly responsible for the oxacillin-resistant phenotype in oxacillin-susceptible mecA-positive Staphylococcus without mec regulators. The initial amount of BlaR1 was determinative for the phenotypic conversion speed under β-lactam exposure.