Spontaneous rosette formation of uninfected erythrocytes around an erythrocyte infected with Plasmodium falciparum is a recently described in vitro phenomenon which is also present in infections with some other malarial species where sequestration of parasite infected erythrocytes is a characteristic. In the present studies, rosetting was established as a P. falciparum virulence factor, the expression of which is modified by a variety of host factors, such as host immunity, ABO blood group and haemoglobin phenotype. The molecules involved in rosetting seem to be distinct from those involved in endothelial cytoadherence, although they are often co-expressed on the same parasitised red cell. Rosette formation was shown not only to be a phenomenon of laboratory-propagated strains, but also to exist in wild clinical isolates from all major malarious areas of the world. In two studies performed in The Gambia, comprising 211 children with uncomplicated or cerebral malaria, a strong association was found between in vitro rosette formation and cerebral malaria, indicating that rosetting plays a role in the pathogenesis of severe P. falciparum disease. Anti-rosetting activity, presumably mediated by antibodies, was found in sera from patients in malaria-endemic areas, and it was demonstrated that such activity was more abundant in individuals with uncomplicated malaria than in those with cerebral disease, suggesting that humoral immunity protects against rosette formation in vivo. It was also demonstrated, by the use of several independent assays, that erythrocytes from individuals with sickle-cell trait, alpha- and beta-thalassaemia trait or with HbE, formed smaller and weaker rosettes than did normal (HbAA) red cells. The results also suggest that microcytosis per se is correlated to impaired rosette formation. Differences in rosetting ability were also seen between red cells of different ABO blood groups, with a diminished rosetting potential in blood group O red cells. Impaired rosette formation may thus contribute to the innate resistance to severe P. falciparum malaria that is known to exist in certain red cell disorders and in individuals of blood group O. Rosette formation was found to be governed by strong adhesive forces, with lectin-like bindings between parasite-derived proteins exposed on the P. falciparum-infected red cell surface, rosettins, and various carbohydrate moieties present on the uninfected erythrocyte. The strongest carbohydrate receptors seem to be contained within the blood group A or B antigens, and the rosettes were abolished by oligosaccharides mimicking these antigens. The binding between infected and uninfected erythrocytes was dependent on divalent cations and was sometimes sensitive to pH.(ABSTRACT TRUNCATED AT 400 WORDS)