Abstract Objective. Co-aggregation and growth inhibition abilities of probiotic bacteria may play a key role in their interference with the oral biofilm. The aim was to investigate the in vitro ability of selected commercial probiotic lactobacilli to co-aggregate and inhibit growth of oral mutans steptococci isolated from adults with contrasting levels of caries. Materials and methods. Mutans streptococci (MS) strains were isolated from caries-free (n = 3) and caries-susceptible (n = 5) young adults and processed with eight commercial probiotic lactobacilli strains. One laboratory reference strain (S. mutans Ingbritt) was selected as control. Co-aggregation was determined spectrophotometrically and growth inhibition was assessed with the agar overlay technique. Results. All probiotic lactobacilli showed an ability to co-aggregate with the isolated MS strains. Statistically significant differences (p <0.05) were found between strains from different individuals when compared with the reference strain. The selected lactobacilli inhibited MS growth, but the ability varied between the strains and was clearly related to pH. No differences were observed between the different MS strains from caries-free and caries-susceptible individuals. Conclusions. The selected lactobacilli displayed co-aggregation activity and inhibited growth of clinical mutans streptococci. The growth inhibition was strain-specific and dependent on pH and cell concentration. The findings indicate that the outcome of lactobacilli-derived probiotic therapy might vary between individuals and depend on the specific strain used.