Patients with primary dengue infection developed dengue 2 virus (D2V) permissive peripheral blood leukocytes (PBL) 2--3 weeks after infection. PBL from healthy individuals with dengue antibody were permissive to D2V in vitro, suggesting that immunologically mediated in vitro D2V permissiveness persists for a relatively long time after recovery from dengue infection. However, PBL obtained from second infection dengue hemorrhagic fever patients did not support D2V growth during the acute phase of illness but did so during convalescence. Leukocytes from dengue-immune patients with typhoid fever or non-dengue viral illness were permissive throughout both acute and convalescent phases of illness although there was tendency for increased permissiveness during convalescence. Acute phase PBL from DHF patients synthesized and secreted dengue neutralizing antibody in culture. Absence of D2V replication in these cultures was strongly, but not completely, correlated with antibody production. Other immunological mechanisms, in addition to antibody, may be operating in vitro or in vivo during acute phase dengue hemorrhagic fever to alter the permissiveness of PBL to D2V infection.