The antimicrobial activity of an aqueous extract of three Brazilian medicinal plants, Alhodomato (Allium sativon), Traoeraba (Comnelina beghlensis) and Goiaba (Psidium guajava), was studied and compared with commercial antibiotics using three different methods: plate count, disk inhibition zone and turbidity techniques. Results indicated that Goiaba leaf extract at a concentration of 8 and 40 mg/ml showed promising results. As Goiaba leaf extract showed good antimicrobial activity against Staphylococcus aureus, nine different strains were tested using these methods. Results showed a complete inhibition of all strains tested at a concentration of 6.5 mg/ml. This indicates that Goiaba leaf extract could be an important source of food preservative and a new source of an antimicrobial agent against S. aureus.