Neutrophil kinetics were studied in seven patients with acute malaria. Six patients with Plasmodium vivax had a mean neutrophil count of 1651 ± 282/ cu mm (range 360-2340); the neutrophil count in one patient with P. falciparum malaria was normal (4600/cu mm). In all of these patients, the numbers of circulating nonsegmented neutrophils were increased, neutrophil half-disappearance times were prolonged, and the total blood granulocyte pools were normal or increased. The marginal granulocyte pools were greatly enlarged but circulating pools were reduced, indicating that the apparent neutropenia was due to an altered intravascular granulocyte distribution. These patients had reduced numbers of mature marrow granulocytes and decreased marrow granulocyte reserves. These data provide a model for one mechanism of neutropenia in man: premature release of marrow granulocytes with a shift of the circulating cells into the marginal pool.