Dengue viruses infect more than 100 million people each year and cause serious clinical manifestations. It is important to understand the replication of these viruses so that therapeutic and/or prophylactic agents may be designed. Dengue virus type 2 nonstructural proteins NS-5 and NS-3 were produced by in vitro transcription and translation of cloned genes. Both proteins possessed RNA-dependent RNA polymerase activity as measured by their ability to convert purified replicative form (RF) RNA to replicative intermediate (RI). The recombinant proteins, however, required one or more cellular protein(s) for their activity. Examination of NS-3 protein sequence revealed heretofore unnoticed sequence similarities with other polymerases.