The neonatal period has been thought of as a window in ontogeny, during which the developing immune system is particularly susceptible to tolerization. In the present study, the classic system for induction of neonatal tolerance to protein antigens was reexamined in mice. The presumably tolerogenic protocol was found to trigger a vigorous T helper cell type 2 (T(H)2) immune response. Thus, neonatal "tolerization" induces immune deviation, not tolerance in the immunological sense. Neonates are not immune privileged but generate T(H)2 or T(H)1 responses, depending on the mode of immunization.