Abstract: When Wistar rats were given intravenously with Poly I:C 5mg/kg 18h before sporo-zoite inoculation, the density of exoerythrocytic forms (EEF) in the rat liver 42h after inoculation was markedly decreased, the sporozoite viability being only 5-12% of that of the controls. However, the size and maturation rate of EEF were all normal. When Poly I:C was given 2h after sporozoite inoculation, the sporozoite viability and the size and the maturation rate of EEF were similar to those of the controls. When the sporozoites were inocubated with rat serum containing interferon for 40 min before inoculation, no detrimental effect on the sporozoite invasion and the development of EEF could be found. These findings suggested that the target of antimalarial action of interferon might not be the EEF itself. It is possible that interferon could minimize the sporozoite number by enhancing the phagocytosis and the biocidal activity of macrophages, especially the Kupffer cells.