The lactate dehydrogenase (LDH) isoenzyme pattern has been determined in human eosinophils isolated from the peripheral blood of healthy donors and patients with parasitic diseases. Almost equal LDH1 and LDH5 values appear to be a characteristic of the eosinophils obtained from healthy subjects. Eosinophilic granulocytes which have been isolated from the peripheral blood of patients with eosinophilia due to parasitic infestations have reduced values of LDH5 accompanied by higher LDH1 as compared with eosinophils from healthy donors. Centrifugation of cells through multiple discontinuous gradients of polyvinylpyrrolidone-coated silica particles made it possible to isolate distinct eosinophil subpopulations of differing densities which showed differences in cell surface markers, K-cell activity and also LDH patterns. Analysis of the lactate dehydrogenase isoenzyme patterns showed that EAG+ and EAC+ eosinophilic leukocytes are characterized by very low values of LDH5.