Although its underlying mechanisms are poorly understood, data comparing each of the four dengue virus serotypes suggest that in vitro antibody-dependent infection enhancement is a reproducible and measurable phenomenon related to other serological measures of antibody-virus binding. Information characterizing infection enhancement may provide clues to disease pathogenesis for dengue and other viruses that exhibit antibody-enhanced infection. We propose criteria for the detection and quantification of in vitro antibody- dependent enhancement of flavivirus infection based on observations using all four dengue virus serotypes, macrophage-like cell lines and human peripheral blood monocytes, and various immune sera and monoclonal antibodies. It is proposed that antibody-dependent infection enhancement is defined by the following findings: (i) significantly increased virus production is measured in quantitative assays at different points on the growth curve; (ii) assays of the virus output of cells infected with mixtures of constant amounts of virus and serial dilutions of the pre-existing antibody source produce characteristic ‘enhancement profiles’ of rising and falling virus output over at least a 10⁻³-fold dilution range; (iii) for each enhancing antibody source the dilution producing maximal infection enhancement is related to other serological measures of binding to the envelope, or another virus component; (iv) infection enhancement is detected with different antibody sources and virus strains (when available) tested over a range of m.o.i.; (v) other causes of enhanced virus production are ruled out.