Four medicinal plants (Quercus infectoria, Kaempferia galanga, Coptis chinensis and Glycyrrhiza glabra) as well as one traditional Thai treatment for aphthous ulcers based on these four plants were tested for antimicrobial activity. MIC values for a range of bacteria and Candida albicans were determined, with both type strains and clinical isolates being used. Antioxidant activity was determined using the ABTS radical scavenging assay. Among four the plants, Q. infectoria showed antimicrobial activity against Staphylococcus aureus with an MIC of 0.41 mg/mL while C. chinensis showed antifungal activity against C. albicans with an MIC of 6.25 mg/mL. Activity was also shown against a range of other organisms including Salmonella typhi, Serratia marcescens, Vibrio cholerae, Vibrio parhaemolyticus, Pseudomonas aeruginosa and Enterococcus faecalis. The antimicrobial activity of the traditional aphthous ulcer preparation (a powder) was comparable to that for the individual plants extracts however incorporation of the powder into a gel formulation resulted in a loss of almost all activity. All extracts, with the exception of K. galanga, also showed good antioxidant activity. This study supports the traditional use of these plants and suggests that they may also be useful in the treatment of other infections.