In this study effect of some separate and combined metabolic compounds produced by lactic acid were tested on food pathogenic and contaminant microorgnasms. For this study, five strains of each Lactobacillus bulgaricus and Streptococcus thermophilus were used. The inhibition effects of the metabolic products (lactic acid, hydrogen peroxide, acetaldehyde, diacetyl) produced by the stains on test bacteria ( Staphylococcus aureus, Escherichia coli) were determined. Also, a random combinations of the strains were prepared and the metabolic compounds produced by these cultures and their antimicrobial activities on tested bacteria were determined. By regarding the metabolic products mainly, the samples of lactic acid, hydrogen peroxide, acetaldehyde and diacetyl concentrations close to these values were prepared and their inhibition effects were examined sperately on two different strains of test bacteria. As a result of this experiment, the inhibition effect of metabolites such as hydrogen peroxide, acetaldehyde and diacetyl could not be determined but it was observed that lactic acid in 5-20 mg/ml concentrations effected test bacteria. In addition to lactic acid (10 mg/ml), the inhibition effects of hydrogen peroxide (3 ppm), acetaldehyde (15 ppm) and diacetyl (1.5 ppm) in 2, 3 and 4 combinations were tested on test bacteria. Whereas it was observed that hydrogen peroxide, acetaldehyde and diacetyl in a combination form had no inhibition effect, diacetyl only in 10 mg/ml and hydrogen peroxide between 0.5-10 mg/ml concentrations showed inhibition effect. Another striking point was that acetaldehyde even in 10 mg/ml had no inhibition effect. Taking into account that lactic acid bacteria can procedure more diacetyl and hydrogen peroxide. The results of this study indicated that the metabolites formed by yoghurt starter bacteria had no inhibition effect and it can be said that the inhibition effect was mainly caused by lactic acid.