Campylobacter jejuni is a foodborne pathogen and a leading cause of diarrhoea worldwide. It is believed that a cholera toxin-like toxin (CTLT) produced by C. jejuni may mediate watery diarrhoea. However, the production of a CTLT by C. jejuni is controversial. A cholera toxin gene (ctx) homologue has not been identified in Campylobacter species. We investigated the identity of the CT cross-reactive antigen from Campylobacter species previously and the results are reviewed here. Filtrates of C. jejuni grown in four different liquid media, reported to promote CTLT production, were tested by Chinese hamster ovary (CHO) cell elongation assay for functional toxin and for reactivity with CT antibody using GM1 ganglioside ELISA (GM1 ELISA) and immunoblotting. Protein sequence of the CT antibody-reactive band was determined by matrix-assisted laser desorption ionization-time of flight (MALDI TOF-TOF). Non-jejuni species (C. coli, C. lari, C. foetus, C. hyointestinalis and C. upsaliensis) were investigated by CHO cell assay and immunoblotting. Filtrates from seven C. jejuni reference strains reported to produce CTLT and from 80 clinical strains were negative in the CHO cell assay. However, filtrates from three reference strains and 16 clinical strains were positive by GM1 ELISA. All strains irrespective of GM1 ELISA reactivity, possessed a 53-kDa protein which reacted with CT antibody by immunoblotting. This band was identified as the major outer membrane protein (PorA) of C. jejuni. CT antibody reacted with a C. jejuni recombinant PorA on immunoblotting. All non-C. jejuni strains were negative by CHO cell assay, but the common 53-kDa proteins reacted with CT antibody on immunoblots. The cross-reactivity of PorAs of Campylobacter species with CT may lead to the erroneous conclusion that Campylobacter species produce a functional CTLT.