Ym1 is an ~45 kDa secreted protein synthesized by activated mouse peritoneal macrophages during the inflammatory response elicited by parasitic infections. Ym1 is a granule protein in neutrophils and has been purified from eosinophilic crystals that form in the bronchoalveolar lavage fluid of mev/mev and SHIP (SH2-containing inositol 5’ phosphatase) knockout mice. The eosinophilic crystals formed in mice are considered to be histologically identical to the Charcot-Leyden crystals found in humans. Ym1 is homologous to T lymphocyte-derived eosinophil chemotactic factor (ECF-L) but elicits only a weak chemotactic response in eosinophils. Ym1 binds to heparin, GlcN oligomers and heparan sulfate. It belongs to a protein family related to chitinases but does not possess chitinase activity.