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The precipitating and neutralizing activities of normal bovine sera with FMDV were studied and compared. Twenty-two out of 79 normal bovine sera gave a positive reaction in micro neutralization tests with FMDV type O, while six did so with type A. In RID tests 32 sera were positive with type O and 28 with type A virus. Almost all of the 79 sera gave a positive reaction in the RID with trypsin treated virus of both types. After three to four fold concentration most sera also gave visible reactions in ID tests when tested against complete virus. When O virus was used the ID patterns produced by most normal sera clearly differed from those obtained with early and late convalescent sera from FMDV infected steers. When type A materials were employed this was also the case but to a lesser extent. The patterns obtained with concentrated normal sera showed, in general a strong line with trypsin treated virus and no line or a weaker one with complete virus. The substances in normal bovine sera precipitating trypsin treated O virus were different from those reacting with trypsinized virus of type A.

Using polyclonal sera raised against foot-and-mouth disease virus in susceptible animals, evidence was obtained for the existence of at least one further important antigenic site in addition to the neutralizing site on VP1 140–160.