It is generally accepted that intraerythrocytic malaria parasites digest hemoglobin to supply the amino acids needed for the synthesis of their own proteins. This view has never been quantitatively tested. In this investigation we have measured the degradation of hemoglobin and the increase in parasite protein content as a function of parasite maturation in cultures of Plasmodium falciparum. Defined parasite stages were obtained either from tightly synchronized cultures or from asynchronous cultures after density-fractionation. We showed that both hemoglobin digestion and total parasite protein content increased with parasite maturation, from the early trophozoite stage onwards, although the total protein content of the parasite remained significantly lower than that of other eukaryotes. The parasite digested up to 65% of the host cell's hemoglobin but utilized only up to about 16% of the amino acids derived from hemoglobin digestion. This large discrepancy is profoundly puzzling particularly in view of the need to detoxify the cell from the large quantities of ferriprotoporphyrin IX and iron released during hemoglobin digestion.