A photoreactive analog of chloroquine, N-(4-(4-diethylamino-1-methylbutylamino)quinolin-6-yl)-4- azi do-2- hydroxybenzamide (referred to as ASA-Q), has been synthesized and shown to mimic the action of chloroquine in possessing substantial antimalarial activity against a chloroquine-sensitive strain of Plasmodium falciparum. As for chloroquine, ASA-Q is less effective at killing drug-resistant strains of malaria, and the resistance can be modulated using the reagent verapamil. ASA-Q has been radiolabeled with Na125I and used as a photoaffinity probe for labeling chloroquine-binding proteins in malaria-infected erythrocytes. Two proteins have been identified with apparent molecular masses of 42 and 33 kDa in both chloroquine-sensitive and chloroquine-resistant strains of malaria. Photoaffinity labeling of the two proteins by iodo-ASA-Q was competitively inhibited by an excess of unlabeled chloroquine. The structurally related antimalarials amodiaquine and quinine also inhibited labeling of the two proteins, while verapamil and doxycycline had no effect. We suggest that the two labeled proteins are the macromolecular targets of chloroquine action in malaria parasites.