A panel of mouse monoclonal antibodies (MAbs) raised against the non-structural glycoprotein NS1 of dengue 2 virus (PR159) was studied for cross-reactivity with the NS1 protein of other dengue virus serotypes and other members of the Flaviviridae using immunoblotting. Most of the 35 anti-NS1 MAbs were found to be specific for dengue 2 virus NS1 (some of which were specific for the native, dimeric form of this protein), but others were found to cross-react within the dengue virus group. This latter group of MAbs, although dominated by MAbs defining a dengue 2 and 4 virus subgroup, also contained some MAbs that were shown to cross-react with both linear (sequential) and conformational epitopes common to the NS1 glycoproteins of all four dengue virus serotypes. Several of these MAbs were also able to cross-react with other flaviviruses, most notably viruses from the Japanese encephalitis antigenic complex. Although cross-reactive epitopes were previously demonstrated on this glycoprotein using polyclonal sera from dengue virus-infected animals and human, this is the first report of the isolation of MAbs which define these determinants and which will allow their further analysis.