Three mature hens were immunized with an Aro- mutant of Salmonella typhimurium beginning with a subcutaneous dose in adjuvant followed by two oral boosters. Isotype-specific antibodies were measured in the white and yolk eggs collected weekly over a period of 230 days. Two hens showed a memory response to the first oral booster, with large increases in egg yolk IgG and smaller increases in IgA and IgM antibodies in egg whites. Smaller amounts of IgA and IgM antibodies were found in egg yolks, and a slight increase in IgG occurred in the whites. One hen showed an increase in serum titers of all isotypes against S. typhimurium. The second hen had high serum titers before immunization was started which did not change. The third hen had a high level of IgM in the white of eggs before immunization was started. This hen showed erratic responses in egg white antibodies following immunization, no increase in IgA or IgM in yolks and only a slight increase in IgG, no increase in serum IgG, and was the only hen with a high level of IgM antibody against S. typhimurium in the bile, conditions reflecting a state of oral tolerance. With the exception of this hen, the results showed that IgA and IgM antibodies were aroused in hens by immunization with an avirulent mutant of S. typhimurium, and that these antibodies were present in the white of eggs from immunized hens.