We have previously reported that rat peritoneal macrophages stimulated with LPS release a factor (MNCF) which induces neutrophil migration that is not blocked by glucocorticoids. The supernatant of macrophage monolayers stimulated with LPS was submitted to affinity chromatography on immobilized sugar columns. We observed that the D-gal binding fraction retained MNCF activity. This fraction, consisting of four protein components, was submitted to chromatography on Superdex 75, yielding a homogeneous preparation of the active component. MNCF has a MW of 54 KDa (gel filtration and SDS-PAGE) and pI < 4.0 (isoelectrofocusing and chromatofocusing). D-gal did not interfere with the behaviour of known interleukins (IL-1 beta, IL-6, IL-8 TNF-alpha), but blocked MNCF activity in an in vitro migration assay. The present results reinforce our previous suggestion that MNCF may correspond to a novel monokine which induces neutrophil migration through a direct mechanism involving the D-gal binding site of the molecule.