A method for immunization of turkeys against exoerythrocytic infection with Plasmodium fallax was devised using formalin-killed merozoites (FKM) from cultures of infected turkey embryo fibroblasts. A single intramuscular injection of FKM combined with complete Freund's adjuvant (CFA) followed at weekly intervals by 2 intravenous (i.v.) injections of FKM in Earle's balanced salt solution (EBSS) (a total of 5.0 X 106 FKM) resulted in 100% survival of turkeys after challenge with infective merozoites when blood stages were drug-suppressed. Two weekly i.v. injections totaling 3.5 X 106 FKM without CFA resulted in 80% survival after challenge. Only 20% of untreated control birds, and about 40% of birds receiving normal cell culture medium overlay, with or without CFA prior to challenge, survived infection. When FKM-immunized birds were challenged with infective merozoites from culture and received no drug suppression of blood stages, parasitemias were similar to those in control turkeys. Also, challenge of FKM-immunized turkeys with parasitized erythrocytes resulted in parasitemias similar to those of control birds. The development of exoerythrocytic stages subsequent to blood-stage infection was suppressed in most FKM-immunized turkeys, while control turkeys eventually died with heavy exoerythrocytic infections in brain capillaries. Past efforts to immunize animals against infection with malaria parasites have been directed at the use of erythrocytic stages or sporozoites as immunogens. Varying degrees of success have been achieved using killed erythrocytic stages of avian malaria parasites (Redmond, 1939; Gingrich, 1941; Jacobs, 1943; Richards, 1966). The use of adjuvants in combination with killed erythrocytic parasites increased the degree of immunity to subsequent infection (Freund et al., 1945; Thomson et al., 1947). The use of killed sporozoites for immunization was effective against mosquito-transmitted avian malaria (Mulligan et al., 1941; Russell and Mohan, 1942), although birds immune to sporozoite challenge were susceptible to infection induced with erythrocytic stages (Russell et al., 1942). Richards (1966) achieved more effective immunization of chickens with killed sporozoites than with killed blood stages of Plasmodium gallinaceum. The development of a culture system for the growth and maintenance of cells infected with exoerythrocytic stages of avian malaria paraReceived for publication 18 September 1973. * Supported by Research Grant AI-08988, and in part by AI-00092, NIAID, U. S. Public Health Service. t Deceased, 27 March 1973. sites (review by Huff, 1969) presents an opportunity for specific immunization with that stage. The results of studies utilizing exoerythrocytic merozoites from infected cell cultures as immunogens against infection of turkeys with P. fallax are herein presented. MATERIALS AND METHODS