While the pathogenesis of eosinophilia-myalgia syndrome (EMS) remains obscure, the ingestion of L-trypophan (LT) and possibly certain constituents in the LT product might be associated. We investigated the effect of chemically synthesized substances, 1,1'-ethylidene bis[tryptophan] (EBT) and its decomposition product, 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (MTCA) recently identified in the implicated LT, on the eosinophil differentiation and the induction of IL-1 and IL-6. EBT and MTCA alone did not support colony formation. However, EBT or MTCA in conjunction with IL-2 induced colony-forming activity containing a small number of eosinophils. In addition, these LT constituents induced a significant level of IL-6 but not IL-1 beta in the mononuclear cells from normal volunteers and a patient with hypereosinophilic syndrome. These results suggest that certain constituents of LT product are associated with the pathogenesis of EMS through the induction of colony-stimulating factors and IL-6, hence giving rise to eosinophilia and inflammation.