Cultures of adult human hepatocytes were infected with sporozoites of Plasmodium falciparum obtained from laboratory-infected Anopheles stephensi. The invasion of the hepatocytes by the sporozoites and their subsequent intracellular transformation to exoerythrocytic forms were monitored up to 72 h by immunofluorescence methods. After 2-h incubation in 2 samples 0.72% (145 and 163) of the sporozoites were attached to the cell layer, of which about 20% were considered to be intracellular. After 24 h changes of shape were observed in some of the sporozoites as they transformed to exoerythrocytic forms. At 24 h these forms still reacted with anti-sporozoite antibodies but there was no such reaction at 52 h. After 72 h, 3 parasites, 6-8 µm in diameter, were detected in 1 culture (0.007% cells infected) by the use of a polyclonal antibody from a P. falciparum-infected patient. The 3-day parasites contained 8-12 nuclei (equivalent to 2-2.5-day schizonts in vivo in man).newline˜S.L. Croft