The ELISA method was used to titrate the humoral immune response in vaccinated mice. When mice were given two doses of a heat-killed salmonella vaccine 6 days apart, there was a steady but low-level increase of antibody synthesis. In contrast, if a booster vaccination was administered 21 days after the primary inoculation, the anamnestic response produced a significantly greater antibody titre, rapidly reaching its peak within 10 days. Such a heightened humoral response induced in the genetically susceptible C57BL/6J mice also coincided with effective protection against an otherwise lethal challenge with Salmonella typhimurium.