According to phylogenetic data, about 100 years ago an avian influenza virus passed the species barrier (possibly first) to pigs and (possibly from there) to humans. In 1979 an avian influenza A virus (as a whole, without reassortment) again entered the pig population in northern Europe, forming a stable lineage. Here it is shown that the early North European swine viruses exhibit higher than normal evolutionary rates and are highly variable with respect to plaque morphology and neutralizability by monoclonal antibodies. Our results are consistent with the idea that, in order to pass the species barrier, an influenza A virus needs a mutator mutation to provide an additional number of variants, from which the new host might select the best fitting ones. A mutator mutation could be of advantage under such stress conditions and might enable a virus to pass the species barrier as a whole even twice, as it seems to have happened about 100 years ago. This stressful situation should be over for the recent swine lineage, since the viruses seem to be adapted already to the new host in that the most recent isolates--at least in northern Germany--are genetically stable and seem to have lost the putative mutator mutation again.