This study has examined in vitro, how exposure to the antimalarial drug artemisinin affects Plasmodium falciparum and its host erythrocytes. Factors examined include: cell morphology, intracellular haemoglobin levels, and haemoglobin catabolism (haemozoin production). To avoid uninfected erythrocytes complicating the study, P. falciparum ring-infected erythrocytes were concentrated to 99% parasitaemia, by saponin haemolysis, before the parasites were grown with or without artemisinin. Without artemisinin, the parasites completed their life cycle in the normal time (40 h), during which a mean of 980 pmol if ferriprotoporphyrin IX from haemoglobin was incorporated into haemozion per 10(6) parasitized erythrocytes, and intracellular haemoglobin level decreased by 90%. Exposure of ring-infected erythrocytes to artemisinin (250 ng per ml of culture medium) inhibited parasite growth completely, haemozoin production by 95%, and decreased the intra-erythrocytic haemoglobin level by 90%; the infected erythrocytes remained intact during the 64 h of study. Haemozoin production was also inhibited when the drug was administered at the trophozoite stage of parasite growth, but the infected erythrocytes haemolysed. These findings may contribute to understanding of antimalarial actions of artemisinin that promote parasite clearance.