Intraerythrocytic malaria parasites meet part of their growth requirements by ingesting and digesting haemoglobin (Hb). To see whether this process is affected by Hb types, the growth and metabolism of Plasmodium falciparum in normal AA, heterozygous AS and homozygous SS erythrocytes were compared in this study. Parasites that have been adapted to continuous growth in AA and AS erythrocytes in vitro were used, and the cultures were incubated in an environment that contained 3% oxygen, 4% carbon dioxide and 93% nitrogen. It was found that exposure of the cultures to this gas mixture caused 5-10% of the AS and up to 90% of the SS erythrocytes to sickle. Parasite growth was essentially normal in the 3 cell types, although multiplication was significantly lower in SS than in AS and AA erythrocytes. Parasite metabolism was evaluated through measurement of haemozoin production. The mean quantity of haemozoin produced by the parasites in AA was comparable to that produced in AS, but significantly higher than that produced in SS erythrocytes. This finding suggests that P. falciparum metabolism is impaired in SS but not in AS erythrocytes. The impairment may be related to polymerization of Hb S.