It was previously observed^2) that the growth of Leuconostoc mesenteroides P-60 was inhibited in the early stage and stimulated in the later stage of incubation by the definite concentration of OMP^*1) in the medium containing DL-alanine. The mechanism of this phenomenon was investigated in the present paper. While consumption of free DL-alanine and other amino acids such as leucine, glycine etc in the medium during the incubation was approximately proportional to the bacterial growth, that of some free amino acides such as glutamic acid, aspartic acid etc and production of ammonia were inversely proportional to the OMP concentration. OMP in the medium was not modified to any substance with vitamin B_6 activity, and it was incorporated in the cells during the growth. When the bacteria grown in the presence of OMP were transferred to fresh medium containing OMP and DL-alanine, the similar growth inhibition and stimulation in early and later stage of the incubation respectively were still observed. Incorporation of PAL^2) into the resting cells was inhibited by OMP, but the inhibition was less observed with cells harvested from the culture containing OMP than with the cells harvested from that without OMP. According to these observations, it would be suggested that OMP inhibits the DL-alanine synthesis, but may not inhibit some other vitamin B_6 enzymes in Leuconostoc mesenteroides. (Received Jan. 12,1971)