Alphaviruses have several features that make them attractive as gene delivery platforms, and vectors derived principally from Sindbis virus (SIN), Semliki Forest virus (SFV), and Venezuelan equine encephalitis virus (VEE), are currently being developed as prophylactic and therapeutic vaccines for infectious diseases and cancer. Alphavirus vectors, termed "replicons", retain the nonstructural protein genes encoding the viral replicase, that in turn programme high level cytoplasmic amplification of the vector RNA. We have developed plasmid DNA and recombinant vector particle delivery systems derived from the prototype alphavirus, SIN. Each system uses RNA polymerase II-based expression of alphavirus genome components and both vector formats are highly efficacious towards inducing robust antigen-specific immune responses in vaccinated animals. To increase the potency of SIN vector particles, which are not known to be lymphotropic, the tropism was re-directed for efficient infection of dendritic cells, both in vitro and in vivo.