The time interval between initial infection with eggs of Hymenolepis nana and onset of a protective host reaction against a challenge with the same parasite has been restudied. These findings confirm earlier reports of an extremely rapid onset of immunity to H. nana. It is concluded that a local, rapidly mobilized antibody reaction by the immediate cells affected could be responsible in spite of the brief period involved. If this can be proved immunologically, it would provide interesting evidence supporting the concept that locally concentrated cellular antibodies are an important host response to tissue invasion by helminths.