The purpose of the present study was to characterize the partitioning of artemisinin into both uninfected and Plasmodium falciparum-infected red blood cells (RBCs). The partitioning of [(14)C](+)-artemisinin into RBCs was studied at four different hematocrit levels and eight time periods. At the optimum time of 2 h, the partitioning process was investigated with eight different drug concentrations ranging from 0.88 to 3.52 microM at 37 and 4 degrees C. The effect of the presence of unlabeled artemisinin on the partitioning of the same concentration of [(14)C]artemisinin was studied. About 35 to 40% of the drug was seen to partition into uninfected RBCs at a hematocrit of 33%, irrespective of the incubation period or the drug concentration used. In contrast, infected RBCs showed an increase in partitioning of the drug with time until saturation was achieved at 1 h. While the partitioning of artemisinin into parasitized RBCs at 37 degrees C was found to be significantly higher than that in nonparasitized RBCs, at 4 degrees C both parasitized and nonparasitized RBCs showed identical partitioning of the drug. The partitioning of [(14)C]artemisinin into parasitized RBCs was completely inhibited in the presence of the same concentration of unlabeled artemisinin. However, no such effect was observed in nonparasitized cells, and no evidence suggesting that binding of the drug in parasitized RBCs is reversible was found. The partitioning of artemisinin into parasitized RBCs was found to be rapid, saturable, temperature dependent, irreversible, and subject to competitive inhibition with unlabeled artemisinin. The results obtained suggest the involvement of carrier mediation in the partitioning of artemisinin across the parasitized RBC membrane. In contrast, simple passive diffusion of artemisinin was seen in nonparasitized RBCs.