Methods are described for the isolation of human eosinophils and basophils from donor blood. Using these cell preparations Charcot-Leyden crystals were found to originate from both eosinophils and basophils when the cells were suspended in hypotonic saline solution. The crystals formed also when saline extracts of eosinophils and basophils were concentrated by ultrafiltration through dialysis tubing. Fractions of eosinophils were prepared and the crystals were obtained from the cytoplasmic fraction but not from the nuclear or granular fractions. Chemical studies showed the crystals to be protein in nature and some evidence is presented which suggests that RNA may decrease the tendency for the protein to crystallize out of solution.