Thirty-two extracts from 22 Mexican medicinal plants of 15 different families were assayed to determine their antibacterial activity against Escherichia coli and Staphylococcus aureus. Seventeen plants showed antibacterial activity, while five plants showed no activity against both bacteria. All of the extracts showed higher activity against Staphylococcus aureus (methicillin-sensitive and methicillin-resistant) than against Escherichia coli, except one. Among the plants examined, Bursera simaruba (L.) Sarg. (Burseraceae), Haematoxylum brasiletto H. Karst. (Fabaceae), Calophyllum brasiliense Cambess. (Clusiaceae), and Mammea americana L. (Clusiaceae) were highly active against Staphylococcus aureus. Coumarins (mammea A/BA and mammea A/AA) and xanthones, namely jacareubin and 1,3,5,6-tetrahydroxy-2-(3,3-dimethylallyl) xanthone, were isolated as the principle compounds from the last two plants.