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The attachment and entry of Plasmodium berghei sporozoites to cultured human lung fibroblast (WI38) or hepatoma (HepG2-A16) cells in vitro has been visualized using an immunoperoxidase technique coupled with light microscopy. Attachment and entry was substantially more frequent with HepG2-A16 cells, and appeared to be mediated by the Pb44 sporozoite surface protective antigen. When sporozoites were incubated with intact monoclonal antibodies to Pb44 or their monovalent Fab fragments, attachment was inhibited, suggesting that this technique may be an in vitro assay of protective immunity. Sporozoites appeared to enter cells actively, rather than by cell phagocytosis. Once within a membrane-bound vacuole, the sporozoite transformed into a trophozoite. It was suggested that Pb44 recognized a specific cell receptor, and that this technique may permit receptor characterization.