In August of 1989 sera from 2115 finishing pigs out of 214 herds in the north of Schleswig- Holstein (Germany) were tested for antibodies against three strains of Influenza A viruses by using the haemagglutination inhibition test. Seroprevalences of A/Swine/Nederland/25/80 (H1N1), A/Philippines/2/82 (H3N2), and A/Port Chalmers/1/73 (H3N2) on herd level were 23.4%, 20.6% and 5.1% respectively. The mean within herd prevalences were 9.6%, 14.7% and 0.6%, respectively. Antibodies against H1N1 and H3N3 strains were simultaneously found in 7.5% of the herds and in 1.6% of the pigs. In 485 (22.9%) of the pigs and in 83 (38.3%) of the herds at least one of the strains was prevalent. In 61 (73.5%) of those herds there was no evidence of a clinical outbreak before testing. A higher risk of infection with influenza virus was associated with high pig density (> or = 200 pigs per km2) and with management factors as greater herd size (> 300 finishing pigs), slatted floors (compared to straw bedding) and all in-all out system (compared to continuous purchase).