Several viruses have been exploited for the development of recombinant vaccine vectors in which to express foreign proteins. Recently, we have described a system utilizing the RNA virus, poliovirus. We have constructed poliovirus genomes in which regions of the capsid have been substituted with gene fragments of the HIV gag and env genes. A complementation system has been designed to encapsidate defective genomes by providing the capsid protein in trans from a recombinant vaccinia virus (VV-P1) (Porter, Ansardi, Choi and Morrow, Virology 1993, 67, 3712-3719). Serial passage in the presence of VV-P1 resulted in the generation of stocks of these encapsidated replicons. Infection of cells with these encapsidated replicons resulted in the expression of the recombinant protein as a fusion protein with the poliovirus capsid proteins VP4 and VP1. In this study, we have utilized encapsidated replicons which express the HIV-1-gag capsid protein (p24) as well as 1.5 kb of the HIV-1 env gene. Stocks of these encapsidated replicons were obtained by 20 serial passages in the presence of VV-P1. In addition, passage of the encapsidated replicons in the presence of poliovirus type 2 Lansing resulted in the encapsidation of the replicons by the capsid proteins provided by poliovirus. The administration of the type 2 Lansing/encapsidated replicons expressing HIV-1 gag into BALB/c mice by intramuscular, intrarectal, or intragastric routes resulted in the generation of antibodies in the serum and secretions against both poliovirus and HIV-1 gag. To prove that the replicons alone are immunogenic, we administered replicons expressing either HIV-1 gag or env to transgenic mice which expressed the receptor for poliovirus type 1. Immunization of these mice by the intramuscular route resulted in the generation of serum antibodies specific for poliovirus as well as for HIV-1 antigens. The results obtained led us to the conclusion that the replicons are immunogenic when given alone or in the presence of poliovirus. These results are important for the use of the poliovirus replicons as a recombinant vaccine vector. © 1995.