Normal volunteers were infected with hookworm larvae Necator americanus. Peripheral blood counts showed a mean of 524 +/- 29 eosinophils/mm3 of blood before infection and a mean of 3,008 +/- 456 eosinophils/mm3 of blood during infection (P less than .01). Absolute numbers of neutrophils did not change. Eosinophils and neutrophils from the infected period were compared with the noninfected state in each subject. The percentage of hypodense eosinophils increased from a mean of 34% +/- 13% to 80% +/- 7% during infection (P less than .05). Superoxide production of eosinophils increased from a mean of 56 +/- 9 to 97 +/- 12 nmol of O2-./10(6) cells per 60 min (P less than .05) during infection. Chemotaxis of eosinophils to Escherichia coli endotoxin-activated serum increased from a mean average distance migrated of 19 +/- 2 micron (P less than .05), whereas neutrophil responsiveness did not change. This is the first report of changes in eosinophil density and stimulation of eosinophil function in normal hosts experimentally infected with hookworm. The data indicate that hookworm infection preferentially increases eosinophil production and activity with little effect on neutrophils.