Injection of a small bacteriophage ϕX 174 into guinea pigs results in an accelerated elimination of phage detectable as early as 24 hours after injection. The immune nature of the accelerated elimination is indicated by its specificity, by the appearance of excess specific serum antibody after phage elimination, and by the prevention of accelerated elimination by 400 r whole body x-irradiation of guinea pigs prior to injection of phage. The early antibody response is considered to be a primary one since an analogous response occurs in newborn guinea pigs, antibody is not detectable in the sera of non-immunized animals, and the second challenge with ϕX stimulates a serum antibody response 100-fold greater than that after primary immunization. The early detection of immune elimination appears to be due, in part, to the small amounts of phage employed, since larger doses of phage delay the time of onset of detectable immune elimination. The early rise of serum antibody in the primary and secondary response appears exponential with a similar rate constant of antibody formation. The rate constant is also independent of dose. These findings have led to the suggestion that during this exponential phase, the relative rate of antibody formation at a cellular level may be constant for a given antigen.