By chromatographic separation on Sephadex gels a peptide, termed the lipopolysaccharide-induced chemotactic factor (LPS-CF), has been isolated from inflammatory exudate. The exudate was obtained from Teflon chambers implanted subcutaneously in rabbits 3 h after LPS from Bacteroides fragilis ss. fragilis had been injected. Three chemotactic peaks were eluted by fractionation of the exudate on Sephadex G-200 columns; one major peak with molecular weight of approximately 16,000 and two minor peaks with molecular weights of approximately 68,000 and 7,000. Refiltration of the major peak on G-75 showed the same elution profile as that found on G-200 columns. By addition of 8 M urea to the elution fluid only the major and the low molecular weight peaks appeared. The molecular weight of the major chemotactic peak was calculated to 16,000 on Sephadex gels, and also using SDS-polyacrylamide gel electrophoresis and equilibrium centrifugation. The chemotactic factor was quite heat-stable and was also non-dialyzable, and freezing and thawing as well as storage at 4 degrees C for several weeks did not impede its activity. This chemotactic factor is probably identical to the cytotaxic fragment split off from C5 upon interaction with LPS.