A simple system that allows expression and packaging of a foreign gene by human parainfluenza virus type 3 (HPIV-3) has been described. First, a cDNA was constructed to encode an internally deleted version of HPIV-3 genome RNA. The viral genes were replaced with a negative sense copy of the bacterial chloramphenicol acetyl transferase (CAT) reporter gene. In vitro run-off transcription with T7 RNA polymerase synthesized an 870 nucleotide RNA that contained the antisense coding region of the CAT gene flanked by the transcription regulatory sequences and the 3' and 5' end extracistronic sequences of the HPIV-3 genome. When introduced into cells that are infected with HPIV-3, this RNA was amplified and the reporter gene was expressed, as measured by the CAT activity in the cell extract. Furthermore, the synthetic RNA was packaged into infectious virions. The addition of two extra nucleotides at the 5' end of the parental trailer region decreased the CAT activity by more than 90%, suggesting a requirement for the intact 5'-regulatory domain in the viral replicative cycle. Interestingly, the addition of one extra nucleotide to the 3' end totally abolished the CAT activity indicating that an exact 3' terminus is critical in this process.