We assessed in vitro the antimicrobial activity of four agents (vancomycin, teicoplanin, mupirocin, and imipenem) which are effective against Gram-positive cocci causally associated with bacterial keratitis, as well as their corneal epithelial cytotoxicity. Minimal inhibitory concentrations inhibiting 90% of strains (MIC90s) against 10 strains each of methicillin-sus-ceptible and -resistant Staphylococcus aureus and Staphylococcus epidermidis, penicillin-susceptible and -resistant Streptococcus pneumoniae, and viri-dans group streptococci were as follows: vancomycin (MIC90s 0.25–2μg/ml), teicoplanin (MIC90s 0.25–4μg/ml, mupirocin (MIC90s 0.12–4μg/mJ), and imi-penem (MIC90s 0.008–0.25μg/ml, except for methicillin-resistant staphylococci with MIC90of 16μg/ml). Cytotoxicity was assayed by uptake of3H-thymidine by rabbit corneal epithelial cell cultures at drug concentrations of 12.5–100 mg/ml for vancomycin and teicoplanin, 1–8 mg/ml for mupirocin and 0.125–8 mg/ml for imipenem, with exposure times of 5, 30 and 60 min. Cytotoxicity was as follows: imipenem