Studies on the metabolism of human eosinophils and their interaction with helminthic parasites have been hampered by the difficulty in obtaining these granulocytes in sufficient purity and quantity. In this report, we describe for the first time the application of the density medium Nycodenz for the purification of eosinophils from peripheral blood. Nycodenz density gradient centrifugation is compared with Percoll and metrizamide gradient centrifugation with special emphasis on cell yield, purity of cell suspensions, and functional integrity of separated cells. Percoll gradients provide purer preparations (mean value 94 +/- 15%) of eosinophils from blood with moderate eosinophilia than Nycodenz or Metrizamide (58 +/- 29 and 88 +/- 13%, respectively). The recovery rates of eosinophil-enriched fractions were low after Nycodenz centrifugation (29 +/- 19%) in comparison to Metrizamide (60 +/- 21%) or Percoll (70 +/- 16%). Eosinophils (as neutrophils), prepared by the 3 techniques, function metabolically (superoxide anion generation) and in assays measuring phagocytosis (S. aureus, zymosan particles). Percoll gradient isolated eosinophils (as neutrophils) adhere to and immobilize larvae of Onchocerca volvulus.