Thirty-three splenectomized Aotus lemurinus griseimembra monkeys with no previous experience with malaria were infected with the Vietnam Palo Alto strain of Plasmodium vivax. The median maximum parasite count was 280,000/microl. Nine splenectomized monkeys with previous infection with Plasmodium falciparum had median maximum parasite counts of 120,000/microl. Splenectomized Aotus nancymai monkeys supported infections at a lower level. Transmission via the bites of Anopheles dirus mosquitoes was obtained in a splenectomized A. lemurinus griseimembra, with a prepatent period of 31 days. It is estimated that between 1.5 x 10(8) and 1.6 x 10(9) parasites can be removed from an infected animal for molecular or diagnostic antigenic studies.