The DIC model (Duodenal Inoculation with ligation of the Cecum in rabbits) was employed to study experimentally induced cholera and the related protective immunity. Duodenal inoculation (DI) without ligation of the cecum with live V. cholerae organisms did not cause any disease symptom but induced protection against subsequent challenges with homologous and heterologous organisms for up to 24 months. After 30 months this protective immunity began to decrease. A similar protective immunity could be induced by administration of the A- B+ derivative CVD101 of V. cholerae strain 395. This type of experiment can only be done successfully with conventional, healthy rabbits held under low stress conditions. A so-called specific pathogen-free rabbit breed was found to be entirely unsuitable. Duodenal inoculation with heat- or merthiolate-inactivated V. cholerae for a prolonged period of time by means of an intestinal osmotic minipump did not induce protection. Injection of heat-inactivated V. cholerae material into the Peyer's patches sometimes led to protection, suggesting that a thermostable antigen, possibly lipopolysaccharide, is one of the major protective antigens. Duodenal administration of a combination of inactivated V. cholerae serotypes Ogawa and Inaba cells and 1 mg B subunit of the V. cholerae enterotoxin by up to three inoculations protected only 3 out of 12 rabbits against challenge. The results obtained on the rabbit model are discussed in relation to the efficacy of this vaccine in human volunteers and in a recent field test.