Infectivity of mutant sporozoites as measured by the prepatent period after infection of mice is strongly reduced. After intravenous injection of sporozoites ~70% of the mice did not develop a blood stage development; the other mice showed a delay of the prepatent period of at least 4 days. Infectivity of sporozoites as measured by invasion of hepatocytes is comparable to that of wild type sporozoites. Invasion of hepatocytes in vitro by sporozoites was normal and liver stages developed into 'mature' liver schizonts. The formation of merosomes was strongly reduced compared to wild type liver schizonts.