Relation of virus particle counts to the hemagglutinating activity of influenza virus suspensions measured by the HA pattern test and by use of the photometric HCU method.

The ratios of the number of virus (n.v.p.) to the hemagglutinating activity were determined for a series of influenza A and B strains using both the photometric HCU method and the HA pattern test. The logarithms of the ratios of n.v.p./hcu ranged from 7.06 to 7.54 and the logarithms of the ratios of n.v.p./HA from 6.87 to 7.56. While the n.v.p./HA ratios of different strains showed significant differences in a number of cases, such differences were not found for the n.v.p./HCU ratios with the only exception of the strain B/Hong Kong/8/73 differing uniformly from the other strains tested. Since the standard deviation of HCU values ranged from 0.006 to 0.039 log units and that of HA titers from 0.076 to 0.272 log units, it was concluded that the differences of n.v.p./HA ratios found were due to the range of variance of HA tests and did not reflect strain specific differences. The degree of purity of virus suspensions was found not to influence measurably the log n.v.p./HCU values.