The role of some well-characterized putative colonization factors (PCFs) in enterotoxigenic Escherichia coli (ETEC), i.e. PCFO159, PCFO166, CS7, CS17 and CFA/III, for colonization of the bacteria in the intestine was studied in a non-ligated rabbit intestine model (RITARD). Intestinal administration of 10(11) organisms of the various strains only resulted in very mild symptoms with loose stools during a few days in most of the animals. Strains expressing PCFO159, CS7, CS17 and CFA/III were shed in the stool for a significantly longer period than PCF/CS-negative ETEC. However, the mean time of shedding PCFO166 positive organisms did not significantly exceed that of non-fimbriated E. coli. All strains that colonized rabbit intestine, as assessed by prolonged fecal excretion, also gave rise to high serum antibody responses against the homologous fimbriae whereas non-colonizing strains failed to induce such responses. This study strongly suggests that several of the recently described PCFs, e.g. PCFO159, CS7, CS17 and CFA/III are colonizing factors and strong immunogens.