Effects of antibiotics on the growth of lactic bacteria, Lactobacillus arabinosus 17-5,L.casei E, L.fermenti 36 and Streptococcus faecalis R, in a complete or controlling synthetic medium were investigated. Aureomycin inhibited the growth of Streptococcus faecalis R at a concentration of 10μg/ml in the controlling medium, but not at the same concentration in the complete medium. Chloramphenicol stimulated the growth of L.arabinosus 17-5 and L.fermenti 36 at a concentration of 0.5-1.5μg/ml in the controlling medium. Other antibiotics tested did not show such an effect.