The morphofunctional state of apudocytes in the gastrointestinal tract and immunocompetent organs (spleen, mesenteric lymph nodes) of mice immunized with chemical bivalent cholera vaccine was studied. The study revealed that the APUD system of the intestine and the argyrophil elements of the immunocompetent organs of white mice gave a response to the oral administration of commercial cholera vaccine. The reaction of the APUD system of the gastrointestinal tract was manifested by a significant increase in the number of apudocytes and their greater synthesizing activity in the immunized animals during the period of maximum immunological transformation of the macroorganism. The immunization of mice with Vibrio cholerae facilitated the maintenance of homeostasis in the macroorganism and prevented appearance of morphological disturbances in its organs and system after subsequent challenge with V. cholerae.