Linezolid is a new oxazolidinone with potent antibacterial activity against Gram-positive cocci; it uniquely inhibits bacterial translation through inhibition of 70S initiation complex formation. The effects of sub-growth-inhibitory concentrations of linezolid on the expression of various structural and soluble virulence factors of Staphylococcus aureus and Streptococcus pyogenes were examined. For S. aureus, strains Wood 46 and Cowan 1 (NCTC 8532) were used to measure protein A, coagulase, alpha-haemolysin (hla) and delta-haemolysin (hld). For S. pyogenes, strain NCTC 9994 was used to measure M protein, streptolysin O (SLO) and DNase. Coagulase was assayed by clotting of citrated rabbit plasma, and hla, hld and SLO by lysis of rabbit, human and horse erythrocytes, respectively. Protein A and M protein were measured indirectly using bacterial susceptibility to phagocytic ingestion of radiolabelled bacteria by human neutrophils. When S. aureus was grown in 1/2, 1/4 and 1/8 MIC, linezolid, coagulase, hla and hld production were impaired. Susceptibility to phagocytosis was changed by growth in the presence of 1/2 MIC linezolid compared with that in its absence (50.8 +/- 4.1% versus 38.9 +/- 2.9%; P <or= 0.05). When S. pyogenes was grown in 1/2, 1/4 and 1/8 MIC linezolid, SLO and DNase production were impaired compared with that of bacteria grown in the absence of the drug; its susceptibility to phagocytosis was also increased (52.8% bacteria ingested versus 37.5%; P <or= 0.05). A reduction in virulence factor expression at sub-MIC linezolid concentrations may be of benefit in the treatment of Gram-positive infections.