The host range and pathology of influenza viruses in ruminants is not yet known. However, based on published results of serological and viral studies the following epidemiological features are documented. Positive serology using the complement-fixation technique has been reported for A2/Japan/305/57, Sw/15, Sw/Shope/58, A/Equi/Prague/57, and B/Johannesburg/59 in the United States and against A and B viruses in Italy. Using the hemagglutination inhibition test, positive findings have been reported for A/PR/8/34, A/equi/Prague, A/Equi2/Lexington, Sw/15, Sw/Shope/58, and B/Johannesburg/59 in the USA against A/PR/8/34, A/FM/1/47, B/Bonn, and B/Roma/1/59 in Italy, and against several strains of A2/H3N2 virus in Rumania, USSR, Nepal, India, and Hungary, even though the single radial hemolysis test is recommended for ruminants. The following influenza viruses have been isolated from cattle: Sw/Shope (H1N1) in Hungary, several strains of H3N2 in the USSR, and two viruses with an unidentified hemagglutinin and a type 2 neuraminidase in Hungary and the USSR. The Russian strain A/calf/Duschambe/55/71 (H3N2) has been recognized as a cattle strain.