Infectivity of supernatant fluids of MLV positive xenograft derived cell lines. Five previously XMLV virus-free cultures (2 NSCLC lines: NCI-H1299 and NCI-H460; two SCLC lines: NCI-H69 and NCI-H1092, and an immortalized human bronchial epithelial cell culture HBEC3K-TR53 [HBEC3]), were infected with the supernatant fluids (250 µl) from 3 XMLV-positive xenograft derived cell lines (LAP C-4, NCI-N417 and 1065met) (Table 1), which had the reverse transcriptase (RT) activities of 5.7 × 104 nU/µl, 3.1 × 105 nU/µl and 1.8 × 105 nU/µl, respectively. The supernatants collected at days 5 and 14 post-infection were examined for the RT activity (see Materials/Methods). The supernatants from the infected NSCLC cell line NCI-H460 showed minimal or no RT activities for all three viruses on day 5, 14 and 18 post-infection (data not shown).