Interactions between animal viruses and antiviral antisera may exceptionally result in an apparent increase in viral infectivity. Halstead and coworkers demonstrated enhanced replication of dengue virus (a Flavivirus, family Togaviridae) in human or simian peripheral blood leucocytes carrying Fc receptors at subneutralising concentrations of antidengue antibody. We have used three continuous cell lines which express macrophage markers to explore the mechanism of this phenomenon. Dengue virus failed to replicate in these cells, but West Nile virus, another Flavivirus, replicated in all three, and we were able to demonstrate reproducibly 50--100-fold enhancement of virus yields in the presence of Flavivirus antisera, the effect also being directly demonstrable in P388D1 cells by increased numbers of virus-induced plaques. The phenomenon of antibody-dependent enhancement of viral replication is not unique to dengue virus, and may have far wider relevance in other viral infections.