Coronavirus defective RNAs (D-RNAs) have been used for site-directed mutagenesis of coronavirus genomes and for expression of heterologous genes. D-RNA CD-61 derived from the avian coronavirus infectious bronchitis virus (IBV) was used as an RNA vector for the expression of chicken gamma interferon (chIFN-ฮณ). D-RNAs expressing chIFN-ฮณ were shown to be capable of rescue, replication, and packaging into virions in a helper virus-dependent system following electroporation of in vitro-derived T7 RNA transcripts into IBV-infected cells. Secreted chIFN-ฮณ, under the control of an IBV transcription-associated sequence derived from gene 5 of the Beaudette strain, was expressed from two different positions within CD-61 and shown to be biologically active. In addition, following infection of 10-day-old chicken embryos with IBV containing D-RNAs expressing chIFN-ฮณ, the allantoic fluid was shown to contain biologically active chIFN-ฮณ, demonstrating that IBV D-RNAs can express heterologous genes in vivo.