This study reports the antimicrobial evaluation of the species most commonly used in Rio Grande do Sul (RS), the southernmost state of Brazil, for treating conditions likely to be associated with microorganisms. A four-stage process of documentation and evaluation was conducted: (a). review of RS ethnobotanical studies; (b). analysis of traditional uses; (c). literature survey on phytochemical and pharmacological data; (d). microbiological screening of selected plants. From the 149 species initially identified, 49 were cited as being used for microbial associated conditions in at least two other regions in RS, and 18 were further selected for screening. The crude methanol extract of these 18 plants were evaluated against seven microorganisms using the diffusion agar test. Extracts from Chaptalia nutans, Cordia monosperma, Echinodorus grandiflorus, Eugenia uniflora, Leonurus sibiricus, Luehea divaricata, Malva sylvestris, Ocotea odorifera, Parapiptadenia rigida, Pluchea sagittalis, Psidium cattleyanum and Senna neglecta were active against at least one microorganism. Although preliminary, these results are useful for rationalizing the use of medicinal plants in established systems of traditional medicine in primary health care.