Although eosinophila is found in many allergic and hypersensitivity diseases, the function of the eosinophil is not clearly established. To evaluate and characterize this function, anticoagulated blood from normal subjects was separated into purified populations of both eosinophils and neutrophils by a modified method for Percoll gradients. With this separation procedure, highly purified populations of eosinophils (95.0% .+-. 2.1%) and neutrophils (97.2% .+-. 0.4%) were obtained. Functional response of these two isolated granulocyte cell types was measured by luminol-dependent chemiluminescence (CL) and superoxide generation to opsonized zymosan and phorbol 12-myristate 13-acetate (PMA). Both the eosinophil and neutrophil peak CL response and superoxide generation to zymosan (1 mg), in the presence of autologous serum (10%), were identical. In contrast, when PMA (10-4 to 100 .mu.g/ml) was the stimulant, eosinophil CL was at least twofold greater than the neutrophil light emission (1,595,741 .+-. 122,435 cpm/5 .times. 105 cells vs. 765,448 .+-. 24,171 cpm/5 .times. 105 cells; n = 6). This same differential in responsiveness was seen in superoxide generation. Thus, under certain conditions the eosinophil''s respiratory burst may be greater than that of the neutrophil, and this differential in metabolic activity may contribute directly to be eosinophil''s inflammatory potential.