To study the role of the erythrocyte membrane in the process of chloroquine accumulation, surface polypeptides were digested with a nonspecific protease from Streptomyces griseus. This treatment activated a saturable process of chloroquine accumulation with an affinity and a specificity similar to those of mouse erythrocytes infected with Plasmodium berghei CS (chloroquine susceptible). Studies of competitive inhibitors of chloroquine accumulation yielded the following approximate values for K(i): amodiaquine, 2 x 10(-7) M; quinacrine, 5 x 10(-7) M; quinine, 2 x 10(-6) M; and mefloquine, 2 x 10(-5) M. Lack of a substrate requirement distinguished this process from the one used by P. berghei and permitted the protease to be used in studies of infected erythrocytes. Protease treatment of erythrocytes infected with P. berghei CR (chloroquine resistant) produced a dramatic transformation. Instead of describing a sigmoid curve, the process of chloroquine accumulation became saturable and substrate dependent, with a K(diss) of approximately 10(-8) M; i.e., protease-treated erythrocytes infected with P. berghei CR now behaved similarly to those infected with P. berghei CS. Coating the erythrocyte surface with albumin completely inhibited the protease-activated process of chloroquine accumulation. These findings are presented as evidence that erythrocyte surface components determine the affinity with which chloroquine is accumulated and thereby determine whether or not the malaria parasite will be susceptible to the drug.