Campylobacter jejuni is a common gastrointestinal bacterial pathogen. Although cytolethal distending toxin (CDT) is proposed to be an important virulence determinant of this pathogen, how CDT(+) and CDT(-) strains differ in their biological properties remains largely unknown. The virulence properties of CDT(+) and CDT(-) strains were studied on HeLa cells and in the suckling mouse model. Presence of the cdtB gene in Campylobacter species was determined by PCR. Five each of CDT(+) and CDT(-) C. jejuni strains were subjected to adherence, invasion and cytotoxicity assay on the HeLa cell line. Bacterial culture supernatants with and without CDT activity were inoculated intragastrically into 2-day-old suckling mice. The mice were sacrificed within 48 h. Histopathological examination of stomach, jejunum, ileum and colon was performed by haematoxylin/eosin staining. cdtB was detected in 88 % and 14 % of C. jejuni and Campylobacter coli strains, respectively. CDT(+) C. jejuni strains adhered to and invaded HeLa cells in significantly higher numbers than CDT(-) strains [CDT(+) vs CDT(-), adherence 2.7 x 10(4)+/-3.5 x 10(4) vs 2.7 x 10(2)+/-1.9 x 10(2); invasion 1.0 x 10(3)+/-1.3 x 10(3) vs 1.4 x 10(1)+/-3.1 x 10(1); P<0.01]. Culture supernatants of all CDT(+) strains demonstrated CDT activity on HeLa cells. Mice inoculated with supernatant containing CDT activity had moderate to severe pathology in different parts of their gastrointestinal tract, with the colon being the major target. Mice inoculated with supernatant lacking CDT activity showed no significant pathology in the gastrointestinal tract. The results demonstrate that CDT(+) C. jejuni strains adhere to and invade epithelial cells more efficiently than CDT(-) strains. CDT is responsible for intestinal pathology and the colon is the major target.