A WIDE variety of solid and hematologic cancers are occasionally accompanied by peripheral-blood eosinophilia,1 but the mechanisms underlying this curious relationship remain obscure. To elucidate one mechanism that may account for cancer-associated eosinophilia, we studied a patient whose waxing and waning pulmonary carcinoma closely paralleled a striking eosinophilic leukemoid reaction. This association led us to hypothesize that the tumor was producing and secreting a hematopoietically active factor that stimulated bone-marrow production of eosinophils. We sought to demonstrate and characterize this putative “eosinophilopoietic” factor in the patient's tumor and serum. We found that even in high dilutions, a buffered saline extract.