To study the contributions of the hemagglutinin-neuraminidase (HN) and the fusion (F) glycoproteins in virus-induced membrane fusion, the HN and F proteins of human parainfluenza virus type-1 (hPIV-1) and Sendai virus (SV) were expressed in HeLa T4+ cells using the vaccinia virus-T7 RNA polymerase transient expression system. Expression of F protein alone did not induce cell fusion. However, coexpression of homologous F and HN proteins resulted in extensive syncytium formation by hPIV-1 or SV glycoproteins, which supports the proposal that both the F and HN glycoproteins are necessary for membrane fusion. To investigate the function of HN in membrane fusion, we coexpressed heterologous combinations of the HN and F proteins of hPIV-1 and SV. No fusion was observed when SV HN and hPIV-1 F proteins were coexpressed. In contrast, the coexpression of hPIV-1 HN and SV F induced extensive cell fusion. These results suggest that specific interaction between HN and F is required to induce membrane fusion. To locate regions that are essential to the fusion promoting activity, chimeric HN proteins of SV and hPIV-1 were constructed. The chimeric proteins coexpressed with the SV or hPIV-1 F proteins indicated that some regions in the middle 62% of HN contribute to the fusion-promoting activity. To determine the role of the transmembrane region of HN on fusion-promoting activity, mutant HN proteins were expressed and their biological activities examined. Mutation of hPIV-1 HN at residue 55 from cysteine to tryptophan did not affect cell binding, neuraminidase activities, or homooligomer formation, but did result in the loss of cell fusion activity. The mutation of the same cysteine residue to glycine retained the fusion-promoting activity, suggesting that a sulfhydryl moiety is not specifically required at position 55, but the structure of the residue that occupies the position is important in fusion-promoting activity.