The effect of ascorbic acid on plasmid-coded antibiotic resistance in Staphylococcus aureus was investigated. Several strains of S. aureus were cultured in the presence of 1 mM ascorbate for 6 h. This treatment induced an increased loss of resistance markers in 4 of 6 strains tested, and agarose gel electrophoresis showed this disappearance of plasmid DNA in ascorbate-induced susceptible colonies. The presence of ascorbate induced a 50-75% decrease in minimal inhibitory concentrations of different antibiotics for resistant strains. When ascorbate is added, formerly subinhibitory concentrations of penicillin or tetracycline have an increased inhibitory effect on resistant strains and even induced the death of 25-93% of the initial population. These results suggest that ascorbate can induce the loss of several plasmids of S. aureus, and that the levels of antibiotic resistance are also affected by the presence of this compound.