A simple assay for listeria lipoteichoic acid was devised. It is based on the property of this substance to attach spontaneously to erythrocytes. Thus coated rabbit erythrocytes were agglutinated by a rabbit antiserum raised towards dead listeria. Inhibition of this passive agglutination was used as a measure of listeria lipoteichoic acid. Cross-reactivity of lipoteichoic acid from Lactobacillus fermenti in the assay and inhibition by simple sugars suggest that the assay is specific for terminal galactose.