Murine replication-competent retrovirus (RCR) is a potential contaminant of biologics prepared in cells of murine origin (for example, murine hybridoma-derived monoclonal antibodies). They are of particular concern in preparations of murine replication-defective retroviral vectors, where recombination events in the packaging cell line can give rise to murine RCR. Presence of murine RCR represents a safety concern for human use because these viruses integrate into the genome, are known to pseudotype human retroviruses, and are associated with tumorigenicity in animals. For example, one study in immuno-suppressed monkeys who received high doses of murine RCR developed lymphomas within 200 days (Donahue, R. E., et al, J. Exp. Med. 176:1125-1135). Using the PG-4 S+L- assay, we are examining parameters affecting sensitivity of RCR detection. For example, we have demonstrated that presence of high titer retroviral vector inhibits detection of RCR. We are coupling our experimental analysis with statistical modeling. The results from these analyses, with additional input from the gene therapy community, are being applied toward the development of new guidelines on detection of murine RCR in biologics.