This study was designed to gain insight into amantadine susceptibility of porcine influenza A viruses isolated in Germany between 1981 and 2001. The 12 studied H1N1, H1N2, and H3N2 porcine influenza virus strains were isolated in chicken eggs and passaged once in MDCK cells. Plaque reduction assays were applied to examine virus susceptibility to amantadine. Genotyping was used to confirm drug resistance. In the results of these antiviral studies, only 3 of the 12 isolates were shown to be amantadine-susceptible. All resistant strains contained the AA substitutions G16E, S31N, and R77Q in the membrane protein 2 (M2). Additionally, L27A was detected in two H1N1 strains. S31N and/or L27A are well-known amino acid substitutions in M2 that confer amantadine resistance. The role of the pig as an intermediate host of avian and human influenza A viruses, the possible involvement of genetic reassortment, and the high incidence of naturally amantadine-resistant porcine influenza A viruses suggest a real risk of emergence of amantadine resistant human viruses. Therefore, drug susceptibility monitoring appears to be warranted for effective application of those drugs.