Plasmodium vivax is the most widely distributed human malarial parasite, there being around 80-90 million cases a year. In some parts of the world, including Latin America and Asia, this is the most prevalent species among the four human malarial parasites. P. vivax cannot be continuously culture in vitro although some progress has been made on this area. Parasites should therefore be obtained from patients or infected monkeys. Although this limitation has somewhat hindered research of this parasite species, it has also resulted in our obtaining of valuable information from natural parasite populations.