The aim of the study was to evaluate the colonizing ability and the invasive capacity of selected Campylobacter jejuni strains of importance for the epidemiology of C jejuni in Danish broiler chickens. Four C jejuni strains were selected for experimental colonization Studies in day-old and 14-day-old chickens hatched from specific pathogen free (SPF) eggs. Of the four C jejuni strains tested, three were Penner heat-stable serotype 2,flaA type 1/1, the most common type found among broilers and human cases in Denmark. The fourth strain was Penner heat-stable serotype 19, which has been shown to be associated with the Guillain Barre Syndrome (GBS) in humans. The minimum dose for establishing colonization in the clay-old chickens was approximately 2 cfu, whereas two- to threefold higher doses were required for establishing colonization in the 14-day-old chickens. Two of the C jejuni strains were shown to be invasive in orally challenged chickens as well as in three different human epithelial cell lines.