The specificity of usage of promoters for replication and transcription by the pneumoviruses human respiratory syncytial virus (HRSV) and avian pneumovirus (APV) was studied using minigenomes containing a reporter gene. When infectious HRSV or APV was used as helper virus, replication could occur only if both the leader and trailer regions (containing the replicative and transcriptional promoters) were derived from the helper virus. In contrast, when the HRSV replication complex was supplied from cDNA plasmids, a minigenome containing either the APV leader or trailer was recognized and substantial levels of replication and transcription occurred. These data suggest that in pneumovirus-infected cells, helper virus functions can discriminate between genomes on the basis of the terminal sequences and that there is an association between the leader and trailer required for productive replication. This association is required only in virus-infected cells, not when replication and transcription are mediated by plasmid-directed expression of the component proteins required for replication and transcription. The possible implications of this are discussed.