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Dengue virus of types 1, 2, and 4 replicated reproducibly in cultures of peripheral blood leukocytes from 18 dengue-immune rhesus monkeys but failed to multiple in peripheral blood leukocytes from 25 nonimmune monkeys. Replication was observed at input multiplicity of infection of 0.1 and 0.01 but irregularly at less than or equal to 0.001. Dengue viruses in permissive leukocytes achieved maximal titers on days 2-4 of culture. Viral replication in peripheral blood leukocytes from animals immune to a different type of dengue virus and in animals immune to two or more types of dengue virus was quantitatively similar to that in peripheral blood leukocytes from homotypically immune donors. Dengue virus permissiveness developed between two and three weeks after primary dengue infections. Leukocytes of animals that developed immunity to dengue virus remained permissive to in vitro infection by dengue virus of types 1 and 2 for prolonged periods, but within six months permissiveness to type 4 decreased. Thus an immunological dependence of in vitro dengue virus replication in simian peripheral blood leukocytes was demonstrated.