We examined the ability of the various human and non-human cell lines to form syncytia upon coinfection with recombinant vaccinia viruses each carrying the HIVenvand the human CD4 gene. We found that three human cell lines and one of three monkey cell lines exhibited syncytium formation, but that one human cell line, two monkey cell lines and all the rabbit and mouse cell lines examined did not. This indicated that factors other than HIVenvand CD4 were participating in syncytium formation and that distribution of these factors was restricted by species and by the cell type in a species.