SummaryRats were rendered tolerant to the bacterial antigenSalmonella adelaideflagellin by intraperitoneal antigen injections in the newborn period. Some rats received a single injection of 100 μg. of flagellin on the day of birth; others a course of twice‐weekly injections for the first 12‐16 weeks of life, the dose per injection usually being 10 μg. At 12 weeks of age, challenge injections of125I‐labelled flagellin or flagella were given, the purpose being to compare the pattern of antigen localization in lymph nodes or spleen with that observed in non‐tolerant control animals.The results showed that most tolerant animals handled the labelled antigen no differently from the controls. Some animals given the twice‐weekly injection course showed enhanced follicutar localization of the antigen. As prior active or passive immunization also causes enhanced follicular localization of antigens, this suggested that in these animals the tolerant state had not been complete, and that traces of antibody directed to minor antigenic components might have exerted an opsonic effect.The theoretical significance of the essentially normal “recognition of foreignness” exhibited by most tolerant animals is discussed.