Living, heat or formalin killed Bacteroides fragilis and a crude preparation of their cell walls were examined by the Boyden technique for chemotactic activity upon guinea pig peritoneal exudate cells. Their relative chemotactic activity ranged from 3.0 to 5.2 compared to an average value of 6.4 for the positive control, an endotoxic culture filtrate of Escherichia coli. A culture filtrate of B. fragilis and an index of 3.7. Miocrogram quantities of cytoplasmic preparations obtained by ammonium sulphate precipitation had chemotactic indices ranging from 2.8 to 6.4, the highest value being displayed by the precipitate formed between 50 and 75% saturation with ammonium sulphate. This fraction retained leucotactic activity after exposure to strong acid and heat. The leucotactic potency of these fractions did not correlate directly with their protein content. Further precipitation of the most active fraction with 80% ethanol revealed that there was little chemotactic activity attributable to polysaccharides. Gas liquid chromatography of a chloroform-methanol extract of the cells which had a chemotactic index of 6.1 revealed the presence of more than thirty fatty acids ranging in carbon length from C8 to C25. These results suggest a role of lipids as initiators of the leucotactic response associated with infections caused by B. fragilis.