(14)C-amodiaquin and (14)C-chloroquine were used to study drug binding by preparations of owl monkey erythrocytes infected either with a chloroquine-susceptible (CS) or with a chloroquine-resistant (CR) strain of P. falciparum. Both of these drugs are derivatives of 4-aminoquinoline, but they differ in their side chains, and there are differences in the way they are handled by preparations of erythrocytes infected with malaria parasites. Although the apparent association constant for the binding of either drug is approximately 10(7) mol(-1), two to three times more radioactivity was bound from (14)C-amodiaquin than from (14)C-chloroquine. Furthermore, there was no apparent difference between CS and CR parasites with respect to (14)C-amodiaquin binding, whereas erythrocytes infected with CR parasites have a deficiency of (14)C-chloroquine binding. This difference in the handling of amodiaquin probably accounts for its superiority in the treatment of the owl monkey and of humans infected with CR P. falciparum.