To investigate the requirements for bovine respiratory syncytial virus (BRSV) cell fusion, the fusion (F), the attachment (G) and the small hydrophobic (SH) glycoproteins were expressed individually or coexpressed, using the vaccinia virus-T7 polymerase transient expression system. The contribution of individual glycoproteins in cell fusion was studied by a reporter gene activation assay. Activation of a reporter gene, beta-galactosidase, was assessed by colorimetric assay of detergent cell lysates or by in situ staining. Quantitative measurements indicated much higher sensitivity compared with analysis of syncytium formation. Our results showed that expression of any individual BRSV envelope gene or coexpression of F + G genes, did not induce significant cell fusion; however, coexpression of F + G + SH genes induced extensive cell fusion. To examine the role of N-linked glycosylation, each of the four potential glycosylation sites were individually removed by mutagenesis. The fusogenic activities of these F glycosylation mutants was examined using the reporter gene activation assay. Our results showed removal of individual carbohydrate chains on F2 subunit had no significant deleterious effects on cell fusion.