Summary Expression of the fimbrial antigen F107 of porcine E. coli strains on agar plates was achieved by microaerobic cultivation. For part of the strains of some types, addition of alizarin yellow and eosin to the agar medium proved to be necessary. Some of these strains reacted distinctly positive only when the small colonies growing between the larger ones on alizarin-yellow agar were tested. The fimbrial antigen of the Swiss strain 107/86 was provisionally designated the F107ab variant and that of the Hungarian strain 2134P and the Czech strain 8813, the F107ac variant. The F107 genetic determinants were found to be often linked with those encoding haemolysin production and are frequently carried by plasmids.