Abstract Owl monkeys ( Aotus lemurinus griseimembra) were immunized against Plasmodium falciparum by infection and drug cure. After challenge, 3 of 4 monkeys developed extended prepatent periods and low grade parasitemias followed by self cure. The fourth monkey did not develop a patent infection. Immune monkey serum passively transferred at the time of challenge conferred immunity to 20 naive monkeys. Immunity was characterized by extended prepatent periods in 19 monkeys, low levels of parasitemia (≤ 1%) followed by self cure in 12 animals, and lack of detectable infection in 3 recipient monkeys, Immune serum collected from monkeys undergoing repeated challenges afforded more protection than serum from singly infected monkeys. However, single doses of hyperimmune serum appeared to be as effective as multiple doses. Normal serum had no effect on the course of infection in 12 monkeys. These studies confirm that owl monkeys can be immunized by infection and cure and demonstrate that this immunity can, in large part, be transferred to nonimmune recipients with serum from immune donors.