The effect of interferon-gamma (IFN-gamma) on dengue virus multiplication in human peripheral blood monocytes was investigated. Enriched monocytes were treated with IFN-gamma and then infected with dengue virus type 2 either directly or in the presence of optimal infection-enhancing levels of antibodies. Pretreatment of monocytes from dengue-immune donors with 100 IU/ml of IFN-gamma caused 12- to 97-fold and 13- to 137-fold reduction of virus yields at 24 hr after infection in the absence and presence of an anti-flavivirus monoclonal antibody, respectively. IFN-gamma also diminished virus yields when infection of monocytes from a donor who lacked anti-dengue antibody was enhanced 40-fold. The percentage of infected monocytes in IFN-gamma-pretreated cultures was similarly reduced. Dominance of the antiviral effect of IFN-gamma in monocytes is in contrast to an augmenting effect previously observed in the promonocytic cell line U937.