Twenty-one isolates were tested for their ability to adhere to and invade HEp-2 cells in vitro. Of the 21 organisms tested, 2 did not invade the HEp-2 cells, and 1 of these did not adhere to the epithelial cells. Campylobacter jejuni clinical isolates were more invasive than the nonclinical strains that were tested. When HEp-2 cells were treated with cytochalasin B, the invasiveness of C. jejuni was reduced, indicating active participation of the host cell in the uptake of these organisms. The number of intracellular C. jejuni isolates decreased when Campylobacter whole-cell lysates were absorbed onto HEp-2 cell monolayers. Experiments were also conducted to identify the functional sites of the antigens responsible for expression of Campylobacter invasion. Oxidation of lysates with sodium meta-periodate significantly affected its inhibitory capacity. This implies that the Campylobacter invasive ligand appears to be dependent upon an intact carbohydrate moiety.