['The objectives of this study are to identify oral commensal species which can inhibit the growth of the main periodontopathogens, to determine the antimicrobial substances involved in these inhibitory activities and to evaluate the influence of environmental factors on the magnitude of these inhibitions.', 'The spotting technique was used to quantify the capacity of 13 commensal species to inhibit the growth of Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis and Prevotella intermedia. By altering experimental conditions (distance between spots and size of spots and concentration of commensal and pathogen) as well as environmental factors (inoculation sequence, oxygen and nutrition availability) the influence of these factors was evaluated. Additionally, the mechanism of inhibition was elucidated by performing inhibition experiments in the presence of peroxidase, trypsin and pepsin and by evaluating acid production.', 'Streptococcus sanguinis, Streptococcus cristatus, Streptococcus gordonii, Streptococcus parasanguinis, Streptococcus mitis and Streptococcus oralis significantly inhibit the growth of all pathogens. The volume of the spots and concentration of the commensal have a significant positive correlation with the amount of inhibition whereas distance between the spots and concentration of the pathogen reduced the amount of inhibition. Inhibition is only observed when the commensal species are inoculated 24h before the pathogen and is more pronounced under aerobic conditions. Hydrogen peroxide production by the commensal is the main mechanism of inhibition.'] Are antimicrobial effects of commensal oral species regulated by environmental factors?