To further investigate the observed differences in XC titers for cells expressing different CAT-1 genes, we assessed infectivity using viral pseudotypes in a single round infectivity assay ( The Spl574 pseudotype is restricted in NIH 3T3 cells as is the Spl574 virus (Tables 1, 2) suggesting that this restriction is entry related. In contrast, the Spl574 pseudotype was not restricted in FerrD2 or FerrM cells although Spl574 virus produces low XC titers in both of these transfectants. This shows that the failure of Spl574 to replicate efficiently in the transfected cells is not entry related and suggests the involvement of factor(s) restricting post-entry stages of Spl574 virus replication in ferret cells.