Summary: Immunization of rabbits with T2 phage results in the early development of neutralizing Ab which, except for its elevated titer, is indistinguishable from the Ab present prior to immunization and designated as normal Ab. Subsequently, Ab is formed which occurs in an additional Cohn fraction and, in contrast to the C′ dependence of normal or early immune Ab, is C′ independent. Moreover, the addition of C′ to phage neutralizing Ab is without effect upon the reactivation of neutralized phage. Finally, both normal and immune sera neutralize phage in accord with the percentage law.