Following an explosive epidemic of A(H3N2) influenza among the human population of Czechoslovakia in 1983, haemagglutination-inhibiting antibodies (titre range 10-640) against strains A/Texas/77, A/Bangkok/79 and A/Philipines 2/83 were detected in 93% of sera collected from 135 pigs on three farms. Only 6.6% of sera were negative. Anti-neuraminidase antibodies were detected at rates of 81% and 23% in two and one of the herds, respectively. Antibodies against A/RNP were demonstrated by the immunodiffusion test in only one of the herds in 10 out of 45 sera tested. This herd was also found to possess antibodies against both envelope antigens of a human A(H1N1) subtype strain. Haemagglutination-inhibition tests with strains A/Hong Kong/68 (H3 N2), A/sw/Shope 15/31, A/sw/Bavaria 2/77 and A/New Jersey 8/76 (H1 N1) were negative in the sera from all three herds.