FIG. 3.

In vitro biological activities of parental and HN chimeric viruses. The HAd activities, NA activities, and fusogenicities of the parental and HN chimeric viruses were examined. The biological activities of these viruses are expressed as percentages of those of the rBC virus, whose activities were considered to be 100%. HAd was measured as the percentage of the hemoglobin released from guinea pig RBCs attached to infected cells. The NA activities of purified parental and chimeric viruses were measured by a fluorometric assay. The fusion index was estimated in Vero cells infected at an MOI of 0.1. Cells were stained with hematoxylin-eosin, and the fusion index was calculated. The fusion index is the ratio of the total number of nuclei to the number of cells in which the nuclei were observed, i.e., the mean number of nuclei per cell. Data are means from three independent experiments.