An improved tetrazolium agar medium for testing sugar fermentation in lactobacilli is described. Basal medium 86 was essentially a modified MRS broth with the omission of glucose. The standard formula was 30 micrograms/ml of 2,3,5-triphenyltetrazolium chloride, 2% sugar to be tested, and 2% agar in medium 86. Plates were incubated anaerobically for 2 days at 37C or 5 days at 30C, depending on the strain. With three strains each of group II and III lactobacilli, colorless, fermentation-positive colonies were clearly differentiated from red, fermentation-negative colonies. For three strains of group I lactobacilli, this medium was not satisfactory because they grew poorly on it unless supplemented with a sugar.