Background: Salivary infection due to Streptococci mutans is considered the main microbiological risk factor for the initiation of dental caries. Aim: To evaluate a semi-quantitative method to assess Streptococci mutans salivary infection and compare it with the existing quantitative method. Patient and methods: Saliva samples were obtained from 650 pre-school children aged 2 to 6 years old using a TYCSB liquid medium for counting Streptococci mutans. Results were compared with quantitative cultures. Results: There was a 99.3% linear correlation coefficient between both methods. The correlation coefficients with dental caries prevalence were 97% for the semi quantitative method and 90% for the quantitative method. Conclusions: The semi-quantitative method to assess oral infection with Streptococci mutans, is accurate enough to be used in population studies and oral infection prevention programs in developing countries (Rev Méd Chile 2003; 131: 412-8).