Malignant diseases of blood system, associated with an eosinophilic syndrome, are accompanied by altered production of some key cytokines regulating proliferation, differentiation and activation events in the cells of eosinophilic lineage, as detected with ELISA approach. An imbalanced secretion of Th2 cytokines, as studied in lymphoproliferative diseases, may be a promoting factor of eosinophilia development in malignant disorders of blood system.