Thirteen dengue virus-specific, cytotoxic CD4+ CD8- T-cell clones were established from a donor who was infected with dengue virus type 3. These clones were examined for virus specificity and human leukocyte antigen (HLA) restriction in cytotoxic assays. Six patterns of virus specificities were determined. Two serotype-specific clones recognized only dengue virus type 3. Two dengue virus subcomplex-specific clones recognized dengue virus types 2, 3, and 4, and one subcomplex-specific clone recognized dengue virus types 1, 2, and 3. Four dengue virus serotype-cross-reactive clones recognized dengue virus types 1, 2, 3, and 4. One flavivirus-cross-reactive clone recognized dengue virus types 1, 2, 3, and 4 and West Nile virus (WNV), but did not recognize yellow fever virus (YFV), whereas three flavivirus-cross-reactive clones recognized dengue virus types 1, 2, 3, and 4, WNV, and YFV. HLA restriction in the lysis by these T-cell clones was also heterogeneous. HLA-DP, HLA-DQ, and HLA-DR were used as restriction elements by various T-cell clones. We also examined the recognition of viral nonstructural protein NS3, purified from cells infected with dengue virus type 3 or WNV, by these T-cell clones. One serotype-specific clone, two dengue virus subcomplex-specific clones, and three dengue virus serotype-cross-reactive clones recognized NS3 of dengue virus type 3. One flavivirus-cross-reactive clone recognized NS3 of dengue virus type 3 and WNV. These results indicate that heterogeneous dengue virus-specific CD4+ cytotoxic T cells are stimulated in response to infection with a dengue virus and that a nonstructural protein, NS3, contains multiple dominant T-cell epitopes.