This chapter describes the development and clinical studies of antiparasitic agents. The natural or acquired immunity to pathologic parasite infections is abolished by immunosuppression, particularly by betamethasone, a corticoid. Identical dihydrofolate reductase enzymes, isolated from P. berghei and P. knowlesi, have been found to be sensitive to pyrimethamine. There is no evidence that inhibition of the enzymes leads to inhibition of DNA synthesis. Labeling experiments show that blood cells infected with P. knowlesi and P. falciparum synthesize coenzyme Q from p-hydroxybenzoic acid and that coenzyme Q8 is apparently the dominant coenzyme Q for P. knowlesi, P. lophurae, and P. cynomolgi. A new cinchona alkaloid synthesis paved the way to various quinine analogs of which the 6'-desmethoxy-7'-chloro-dihydroquinines were found to be 4 times as potent as natural quinine against P. berghei in mice. The 5-nitroimidazoles have generally proven to be superior to the 4- and 2-nitroimidazoles as antiparasitic agents but the latter seem to have better antibacterial properties. © 1972, Academic Press Inc.