During the past 5 years, a considerable number of known and new xanthones from the Calophyllum species of Sri Lanka have been isolated and characterized. We have investigated the antimicrobial activity of Calophyllum xanthones, with a special reference to methicillin-resistant Staphylococcus aureus (MRSA). These activity studies were carried out using the agar plate method. Calozeloxanthone, a xanthone which has been isolated from C. moonii and C. lankensis, showed the highest activity against methicillin-resistant S. aureus (MRSA) strains at a concentration of 8.3 microg/ml. Hence, calozeyloxanthone appears to hold promise as an antimicrobial agent in the treatment of infections with S. aureus, including methicillin-sensitive S. aureus (MRSA), and should be investigated further.