The sera of adult BALB mice showed increased neutralizing activity within 6 hr after the intraperitoneal injection of 1010 actinophage MSP8. Moreover, as few as 2 × 105 phage particles were sufficient to elicit a measurable immune response. Because most serum samples from normal mice contained detectable, specific antibody for coliphage T2, coliphage øX174 or actinophage MSP8, the observed increase in neutralizing activity after injection of phage was not considered a primary response. Sera from animals receiving a single antigenic stimulation displayed phage-neutralization kinetics different from hyperimmune sera. Activity of both early and late sera was in the γ-globulin fraction.