Campylobacter jejuni is an important human enteropathogen worldwide. Chickens are the major reservoir and source of campylobacter infection. Ten clinical isolates from human and five chicken strains were tested for the adherence, invasion and cytotoxin assay in HeLa and HEp-2 cells. All human strains adhered to both the HeLa (10(3) to 3 x 10(4) bacteria/mL of cell lysate) and HEp-2 cells (2 x 10(3) to 4 x 10(4) bacteria/mL of lysate). All chicken strains also adhered to the HEp-2 cells (10(2) to 10(3) bacteria/mL), but only two strains adhered to the HeLa cells. Six clinical and none of the chicken strains invaded the mammalian cells. Both the adherence and invasion were better observed in HEp-2 than in HeLa cell lines. All three isolates from patients having invasive diarrhoea and only one strain from a patient having watery diarrhoea produced cytotoxin. All three invasive strains also adhered to polystyrene surface after the localised destruction of the HEp-2 cells, a phenomenon not reported earlier. Adherence was markedly inhibited by the whole cell lysate and the acid glycine extracts, and the results were comparable. This study indicates that the clinical isolates of C. jejuni are more virulent than the chicken strains, HEp-2 is better for the adherence/invasion assay and HeLa is better for cytotoxin assay. The acid glycine extracts probably contain the key adhesins for C. jejuni.