Non-viral self-replicating vectors based on defective viral genomes have been developed for a number of different alphaviruses including Semliki Forest virus (SFV), Sindbis virus (SIN) and Venezuelan equine encephalitis virus (VEE). These vectors can be used for gene delivery as naked RNA or DNA. Recombinant alphavirus RNA can be synthesized in vitro from plasmids containing the alphavirus replicon under the control of a prokaryotic promoter such as SP6 or T7. These self-replicating RNAs have been able to induce protective immune responses in vivo, probably due to the high level of expression of the recombinant antigen in the transfected cells. However, alphavirus vectors based on the direct delivery DNA are probably a better choice due to their higher stability and lower production cost. In these vectors, the alphavirus replicon is placed under the control of a RNA polymerase II promoter. These vectors are more efficient than conventional plasmids in inducing both humoral and cellular immune responses in small animals, allowing the use of significant smaller amounts of DNA for immunization. In addition, due to the transient nature of the alphavirus replicons, possible problems associated with DNA integration into host chromosomes are eliminated.