This investigation was designed to investigate mechanisms underlying oral immunization in humans after ingestion of the ribosomal vaccine D53. Immunofluorescence and ELISA (Enzyme-Linked ImmunoSorbent Assay) spot techniques were used for peripheral blood studies. The first part of the investigation was a double-blind placebo-controlled study of 12 healthy volunteers; counts of cells containing immunoglobulins and cells producing specific antibodies were higher in the individuals given the oral ribosomal vaccine than in the placebo-treated controls. In the second part of the investigation, analysis of the kinetics of apparition of the immunoglobulin-containing and specific antibody-producing cells suggested prompt stimulation of Peyer patch B lymphocytes following ingestion of the vaccine. Lastly, a study of 5 children given the vaccine on a long-term basis demonstrated increased counts of both above-described cell types after one month treatment.