Antigenic chimeric viruses in which the structural genes of dengue virus type 4 (DEN4) have been replaced with those derived from dengue virus type 2 (DEN2) have been created and evaluated as a first step in generating a live attenuated tetravalent dengue virus vaccine. Specifically, the capsid, membrane precursor, and envelope (CME) or the membrane precursor and envelope (ME) gene regions of DEN2 were substituted for the corresponding genes of wild-type rDEN4 or vaccine candidate rDEN4delta30 which contains a 30 nucleotide deletion in the 3' untranslated region. The two DEN2/4 chimeric viruses lacking the delta 30 mutation were highly attenuated in tumor-bearing SCID-HuH-7 mice, mosquitoes, and rhesus monkeys, indicating chimerization with either the CME or ME regions lead to attenuation. In mosquitoes and SCID-HuH-7 mice, addition of the delta 30 mutation to the chimeric viruses resulted in comparable or only slightly increased levels of attenuation. In rhesus monkeys, addition of the delta 30 mutation rendered the CME chimeric virus non-infectious, indicating that the attenuation resulting from chimerization and the delta 30 mutation were additive for these animals. In contrast, the attenuation in rhesus monkeys of ME chimeric virus was not significantly modified by the addition of the delta 30 mutation. The satisfactory level of attenuation and immunogenicity achieved by the ME containing DEN2/4delta 30 chimeric virus, as well as its very low infectivity for mosquitoes, make it a vaccine candidate suitable for evaluation in phase I clinical trials.