Background The growing interest on usage of probiotic lactobacilli in maintaining oral health has posed number of questions on its probable side effects. One such consideration could be an increased acid production in dental plaque, in turn leading to dental caries. Thus, the aim of this study was to comparatively evaluate the lactic acid producing ability of L. acidophilus and L. plantarum with and without dental plaque. Material and Methods The study consisted of five groups: 3 control groups ( Supragingival plaque, L. acidophilus and L. plantarum ) and 2 test groups (Supragingival plaque with L. acidophilu s and Supragingival plaque with L. plantarum ). 26 samples for each group were collected and their baseline spectrophotometric values were recorded. The acid production was initiated by adding 25?l fructose (10%) and stopped by centrifugation for 2 min. The concentration of the lactic acid produced was determined with the aid of COBAS INTEGRA 400 plus. Results On comparison of Lactic Acid estimation in mg/dl, the mean values of Plaque group was the highest followed by Plaque +L acidophilus, Plaque +L plantarum, L acidophilus and least in L plantarum. The posthoc analysis shows that the comparison of Group 1 (Plaque) and Group 2 (Plaque +L acidophilus) is statistically Significant results between all the groups except between the Plaque +L acidophilus and Plaque +L plantarum group. Conclusions The lactic acid producing ability of pure suspensions of L.acidophilus and L.plantarum and the lactic acid producing efficiency becomes more when they are added to the supragingival plaque. Key words: Probiotic, lactic Acid, plaque.