Examination of the culture fluids from a number of strains of oral streptococci and latobacilli has shown the presence of an erythrocyte-sensitizing antigen with the properties of lipoteichoic acid. The antigen was isolated from the culture fluids of Lactobacillus casei and Lactobacillus fermentum and characterized chemically and serologically, For other strains, serological evidence for the presence of lipoteichoic acid depends on the reactivity with antiserum specific for the glycerol phosphate backbone. The relative concentrations of the antigen in culture fluids from different organisms, in culture fluids from different stages of growth, and in extracts of organisms was estimated by determining the maximum dilution that fully sensitized erythrocytes; the culture fluid titer, which is the reciprocal of the dilution, varied from 4 to 320. Strains of Streptococcus mutans were generally characterized by a high level of extracellular lipoteichoic acid, the amount being greater than that detectable in cell extracts; this conclusion was confirmed by using the quantitative precipitin method. A high-molecular-weight fraction obtained from S. mutans BHT culture fluid was effective in sensitizing erythrocytes at a concentration of 1 mug/ml, compared with 2 mug/ml required for cellular lipoteichoic acid from L. casei. The detecting procedure depends on the teichoic acid sensitizing erythrocytes but, as shown with L. fermentum, low-molecular-weight nonsensitizing teichoic acid may also be present in culture fluid.