This paper describes the ultrastructure of intracellular elongated, transitional and coccoid forms of Campylobacter jejuni, in irradiated mouse jejunum infected both in vitro and in vivo and in cultured human skin fibroblasts. Jejunum of irradiated mouse incubated for 1 hour under conditions favorable to the organisms showed minimal tissue degeneration. The intracellular organisms in this material were free cytoplasmic forms showing inner membrane degeneration, loss of cytoplasmic granules, and absence of flagella. The diameter of the coccoids was up to four times that of the elongated forms, as in plate cultures. Intracellular organisms were not found in challenged unirradiated controls, indicating that irradiation of mouse cells may be required for intracellular infection with human strains of C jejuni. In contrast, challenged human fibroblasts contained typical elongated organisms in cytoplasmic vacuoles. These findings are discussed with reference to Campylobacter strain, host resistance, and "natural" animal and human Campylobacter infections.