Freshly drawn AA and CC red cells were more suitable for in vitro development of Plasmodium falciparum than red blood cells (RBC) stored for 13 days before use. Growth rate inhibition in CC red cell cultures reached 31% in freshly drawn red cells and 57% in aged red cells of the same donor. Ultrastructural studies of CC cells revealed very important irregular cavities sometimes occupied by a granular content. Parasites in CC cells were generally normal but occasionally showed signs of functional impairment. P. falciparum growing in CC red cells was less sensitive in vitro to chloroquine than in AA red cells. This phenomenon may be explained either by the type of the hemoglobin of the host cell or to abnormal haematological parameters of the HbC homozygote donor, particularly the high proportion of neocytes. As metabolism of reduced glutathione is higher in young RBC and as chloroquine lyses parasitized RBC by reducing the regeneration capacities of this compound, the increased rate of young RBC in the CC red cell population was probably related to the decreased chloroquine sensitivity of P. falciparum growing in these cells.