A rapid and precise method is described for determination of absolute numbers of hemagglutinating particles present in virus suspensions. The method involves spectrophotometric measurement of the number of red cells dimerized by the virus under conditions in which formation of larger aggregates is precluded.

Results of this procedure are compared with those of titrations based on egg infectivity and the polystyrene microsphere electron micrography for influenza virus; and with plaque count titrations on chick embryo cell monolayers in the case of the virus of Newcastle disease. The agreement is well within the limits expected in view of the different quantities which the various procedures measure.

The conversion factor for transforming titre in hemagglutinating units based on the pattern procedure into absolute number of agglutinating particles is the same for influenza and Newcastle disease virus.

The precipitation of red blood cells by Newcastle disease virus at 23°C. is optimal at pH's between 6.2 and 5.8.

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