Plaque development assay showing that KD1-SPB plaques form more slowly in 293 and A549 cells than do KD1 plaques. The titers of CsCl-banded stocks of KD1-SPB and KD1 were determined using standard methods (51) on 293-E4 or 293 cells (A) or on A549 cells (B). The data are plotted as the number of plaques seen on any day of the plaque assay as a percentage of the number of plaques seen on the final day of the assay (53). The final titers were as follows (PFU per milliliter): KD1 on A549 cells, 4.1 × 1010; KD1-SPB on A549 cells, 5.7 × 109; KD1 on 293 cells, 2.8 × 109; KD1-SPB on 293 cells, 3.5 × 108; KD1 on 293-E4 cells, 2.1 × 109; KD1-SPB on 293-E4 cells, 1.9 × 108.