OBJECTIVES To determine in vivo association between Aggregatibacter actinomycetemcomitans (Aa) and Streptococcus mutans (Sm) in aggressive periodontitis patients (AgP) and the in vitro influence on Sm of saliva and of Aa strains isolated from individual Aa-positive patients. MATERIALS AND METHODS Clinical indices and saliva samples were taken from 30 AgP patients. Aa and mutans streptococci levels were determined. Antibacterial effect of saliva from 12 Aa-positive patients, and their individual Aa strain, was checked turbidimetrically in vitro on Sm. RESULTS Aa salivary level was inversely correlated with levels of mutans streptococci and directly correlated with pockets of ≥7 mm. During exponential growth phase: (i) All Aa-positive and Aa-negative saliva samples showed no significant influence on Sm growth. (ii) Each individually isolated Aa strain presented significant inhibitory effect on Sm growth. During stationary growth phase, all the above demonstrated an inhibitory effect on Sm growth, with significantly greater influence of Aa individual strains. CONCLUSION Saliva of each AgP Aa-positive subject had an inhibitory effect on Sm growth, which is most likely derived from Aa bacterial physiology. This research raises the possibility that suppression of Aa due to periodontal treatment may increase Sm levels and hence caries incidence.