SummaryPlasmodium falciparumcultures were synchronized using three lytic treatments with sorbitol. Schizonts from these cultures were used in a rapid, highly sensitive assay of invasion of erythrocytes by merozoites. The parasitaemias in recipient cells after invasion were determined by flow cytofluorimetry after staining with the dye 33258 Hoechst. Invasion of erythrocytes was shown to be reduced by serum from a patient with malaria. The assay is suitable for rapidly screening large numbers of samples, such as monoclonal antibodies.