Charcot-Leyden crystals (CLC) are currently believed to be unique to the eosinophil and a hallmark of active eosinophilic inflammation or proliferation. The distinctiveness of the CLC to the eosinophil was questioned in 1965 by Archer and Blackwood (9), but their demonstration of CLC formation in basophils was ignored and later dismissed (1) as being the result of eosinophil contamination of basophil-enriched cell suspensions. We reexamined this question and showed that basophils obtained from the peripheral blood of normal individuals form CLC and that basophils contain a protein that is immunochemically indistinguishable from eosinophil CLC protein. These conclusions are based upon the findings that (a) crystal formation in basophils was demonstrated by specific histochemical staining of crystal-containing cells in highly enriched basophil suspensions prepared by fluorescence-activated cell sorter (FACS) purification of surface IgE-positive cells, (b) that enrichment for surface IgE-positive cells (primarily basophils) by the FACS also enriched for cells staining positively by immunofluorescence for eosinophil CLC protein, and (c) that CLC protein was measured by radioimmunoassay in cell extracts prepared from purified basophil suspensions containing 97-99% basophils and absolutely no contaminating eosinophils. These basophil extracts contained a protein immunochemically indistinguishable from eosinophil CLC protein. Based upon these findings, the CLC or the protein comprising the crystal (lysophospholipase) can no longer be considered as distinctive to the eosinophil. We must now consider the possibility that the presence of CLC in tissues, sputum, or stool may also represent basophil involvement in disease processes.