The cell lines evaluated for the ability to support virus replication included rat epithelial L2 and RLE, mouse epithelial LA4, mouse macrophage RAW 267.4 and J774A.1, and primate epithelial A549, BS-C-1, and HEp-2. All were inoculated with PVM on day 0 (MOI = 0.02, 10 4 pfu per 5 × 10 6 cells). On day 7, virus titer in the culture supernatants was determined by standard plaque assay [12] . Although pneumoviruses maintain strict host-pathogen specificity in vivo, we determined that PVM replicated to a limited extent in vitro (< 10 3 pfu/ml supernatant) in each of the aforementioned cell lines. The mouse monocyte/ macrophage RAW 264.7 cell line (established from a tumor induced by Abelson murine leukemia virus) generated the highest virus titers (10 4 pfu/ml) under culture conditions described. Cells of the RAW 264.7 line also support replication of other unrelated virus pathogens, including murine hepatitis virus and Japanese encephalitis virus [15] [16] [17] .