The in vitro antibacterial activities of the biphenyl aucuparin and xanthones, obtained from the dichloromethane extract of the leaves of Kielmeyera coriacea (Guttiferae) were tested against Gram-positive and Gram-negative bacteria. In addition, time-kill studies were performed to determine if aucuparin had bactericidal activity. It is not known whether the aucuparin found in K. coriaceae is due to de novo synthesis in response to diverse forms of stress, is already present in the plant, or is a combination of both. Aucuparin and 1,3,7-trihydroxy-2-(3-methylbut-2-enyl)-xanthone showed antimicrobial activities against Bacillus subtilis with MIC values of 3.12 mug/ml and 12.5 mug/ml, respectively, and aucuparin against Staphylococcus aureus with a MIC value of 12.5 mug/ml. In contrast to the relatively low MICs for Gram-positive bacteria, Gram-negative bacteria tested were not inhibited by aucuparin at concentrations greater than or equal to100 mug/ml. The kinetics of bactericidal activity were evaluated against S. aureus at six concentrations of aucuparin (0.25x, 0.5x, 1x, 2x, 4x and 8x the MIC). The rate of bacterial killing was dependent on the concentration of aucuparin, with more than 10(5) organisms/ml being eradicated within 8 h at the highest concentration studied, Bacterial culture was monitored for up to 24 h, and no regrowth was observed.