1,1'-Ethylidene-bis-(L-tryptophan) is a potential impurity in L-tryptophan commercial products, promoting EoL-3 eosinophilic leukemia cell proliferation, enhancing eosinophil cationic protein release from human peripheral blood eosinophils, and boosting IL-5 production in human T cells. At 40 µg/kg, it triggers fascia thickening, mast cell infiltration, inflammation, and fibrosis in mouse superficial adipose and panniculus carnosus tissues, serving as an eosinophilia-myalgia syndrome research model.