To understand the mechanisms involved in maintaining long-term immunological memory following mucosal immunizations, we determined the quality of serum hapten-specific immunoglobulins (Ig) and localized Ig-secreting cells (SC) of various isotypes in acute, persistent/resting memory and effector memory phases following oral versus intra-muscular (IM) immunizations. In the acute phase, both oral and IM immunizations induced high avidity Ig. However, in the persistent/resting memory phase, oral immunizations induced low avidity Ig while IM immunizations induced high avidity Ig. Following oral immunizations, in the persistent/resting memory phase, hapten-specific IgM titers in serum and IgM-SC in bone marrow (BM) dominated the immune response, suggesting an important role for IgM in the maintenance of memory.