Recombinant viruses are especially useful for gene transfer and protein expression in vivo and in vitro. During viral vector production, particles may be generated which are replication competent through recombination with host sequences or by contamination. The probability of producing replication competent virus, although low, increases with each successive amplification. Therefore, development of a sensitive assay for detection of replication-competent recombinant viruses in large-scale vector preparations is needed.