The effect of crystalline and of crude trypsin on the hemagglutinins of 35 strains of influenza A and B and one strain of swine influenza virus has been determined under different test conditions. The swine influenza virus was completely resistant in all tests. The susceptibility of the strains of human origin varied with the medium in which the virus and enzyme were contained and with the kind of trypsin preparation employed. Under the most favorable conditions the hemagglutinins of all human strains were at least partially inactivated. By proper selection of test conditions patterns of trypsin-sensitivity were established which, in general, corresponded to the antigenic type or subtype of the virus.

With the exception of PR8, the pattern of trypsin-sensitivity of the A strains isolated before 1937 was, on a group basis, indistinguishable from that of the A1 or A-prime strains, and the pattern of PR8 and the A strains isolated from 1940 to 1945 was similar to but readily distinguishable from that of the A2 or Asian strains.

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