Human immune response to a secondary immunization by the oral route was studied by enumerating specific antibody-secreting cells (ASC) in the peripheral blood, believed to reflect the local immune response in the mucosa. The volunteers had been immunized 1-2 years earlier with live Salmonella typhi Ty21a given orally as three doses of vaccine in enteric-coated capsules (3 x E); their ASC and serum antibody responses have been published. In the present study 17 of the same volunteers received one booster dose of the same vaccine (B-E). Specific ASC appeared in the blood of all volunteers after vaccination, peaked on day 5 and faded away thereafter so that on day 9 specific ASC were detected in only seven subjects. The ASC responses to the single booster dose were significantly higher than those to one dose in primary immunization and at least as high as those of the same volunteers to three doses in primary immunization. Serum antibody responses were not seen in any of the vaccinees after secondary immunization, whereas after primary immunization 60% of these subjects responded. This study shows the presence of immunologic memory also in respect of the human ASC response, and confirms the separate nature of ASC and serum response.