A potential impurity found in commercial preparations of L-tryptophan; increases the proliferation of EoL-3 eosinophilic leukemia cells, release of eosinophil cationic protein from isolated human peripheral blood eosinophils, and IL-5 production in isolated human T cells; induces fascia thickening and increases mast cell infiltration, inflammation, and fibrosis in superficial adipose and panniculus carnosus tissues in a mouse model of eosinophilia-myalgia syndrome at 40 UG/kg