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Serum levels of specific IgG and the sensitization of peripheral blood T-lymphocytes were measured in guinea pigs after single-dose antigenic sensitization by two routes-, intratympanic and intradermal injection. Keyhole limpet hemocyanin (KLH) served as the antigen. Intratympanic injection of antigen resulted in much lower levels of circulating anti-KLH IgC than intradermal injection. When KLH was conjugated with alum to produce nonspecific inflammation and serve as an adjuvant, the intratympanic route was considerably enhanced, but remained much less effective than the intradermal route. Development of an IgG response was also somewhat less rapid following intratympanic than following intradermal administration. Marked sensitization of circulating T-lymphocytes was seen after intradermal injection of alum-precipitated KLH. A much weaker, though still positive, response was seen after intradermal injection of KLH alone and with the intratympanic injection of alum-precipitated KLH. No T-lymphocyte sensitization could be detected after intratympanic injection of KLH alone. It was concluded that the afferent limb of both humoral (IgG) and cell-mediated immunity was operative in the middle ear. Therefore, the middle ear does not represent an immunologically “privileged” site. On the other hand, the afferent limb from the middle ear appears to operate less effectively and rapidly than that from the dermis. This observation is consistent with observations in other mucosal systems.