A comparative study on the infectivity and biological activities of parainfluenza virus type 2 revealed that one strain (Toshiba) showed c.p.e. with cell fusior and produced plaques in Vero cells, an established line of African green monkey kidney cells. Another strain (62-M786) of this virus, however, showed minimum c.p.e. and produced almost no plaques in Vero cells, although c.p.e. appeared and plaques were observed following addition of trypsin. The infectivity titre of the latter strain, estimated by 50% tissue culture infectious doses (TCID), was increased 10 times and plaque counts more than 6400 times by addition of an adequate concentration of trypsin; however, trypsin did not affect the infectivity of the former strain. Trypsin also increased the haemolytic activity of strain 62-M786 but not that of the Toshiba strain. These results show that the two isolates of parainfluenza virus type 2 were affected very differently by trypsin as regards infectivity, cell fusion and haemolytic activity.


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