Trypsin treatment of adherent human monocytes greatly reduced or eliminated the ability of these cells to support dengue virus replication. However, addition of dilute (nonneutralizing) antibody to the inoculum and the culture medium resulted in viral yields similar to those from monocytes not treated with trypsin. These results suggested that viral entry was facilitated by phagocytosis of immune complexes via Fc receptors on the monocytes. This concept was tested by (i) pretreating monocytes with aggregated gamma globulin, which resulted in a 40-fold reduction of viral yields after infection with dilute antibody-virus complexes and (ii) forming an immune complex with virus, antivirus F(ab')2 fragments, and rabbit anti-human Fab. Whereas F(ab')2 fragments alone would not enhance virus replication in trypsin-treated monocytes, the immune complex containing a rabbit Fc piece did increase the yield of dengue virus. These results suggest that dengue virus can infect a cultured monocyte in two ways: (i) through a viral receptor that is trypsin sensitive or (ii) through an Fc receptor that is not trypsin sensitive.