Employing rabbit immune serum, 10 flaviviruses known to be present in South Africa could be divided into 5 serological subgroups. The subgroups conform to the general pattern described for the group. Sera from experimentally infected calves and lambs were monospecific in neutralization tests, but cross-reacted in haemagglutination inhibition tests. These results suggest that sheep and cattle sera from the field can best be tested by microneutralization tests. The greater sensitivity of embryonated hen's eggs for some viruses and of one-day-old mice for other viruses necessitates the employment of both systems for the isolation of flaviviruses from field specimens.
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