Abstract DBA/1 mice were immunized by an intraperitoneal injection of 0.2 to 1.0 µg of ovalbumin precipitated with aluminum hydroxide gel and the antibody response was observed. Both reaginic and γ1 antibodies were detected in the serum 10 days after the immunization and the antibody formation persisted for more than 1 month. The animals showed a secondary reaginic antibody response upon booster injection. An intravenous injection of 80 µg N of mouse anti-ovalbumin IgG antibody into the immunized mice within 24 hr after primary immunization prevented both reaginic and γ1 antibody responses for a period of over 3 weeks, during which time passive antibody was detectable in the serum. Evidence was obtained that an essentially normal immune response followed after the passive antibody was catabolized. Both reaginic and γ1 antibodies were detected in the serum 40 days after immunization. An increased interval between immunization and passive antibody resulted in progressively decreasing efficiency of inhibition of the antibody response. Passive administration of IgG antibody did not terminate pre-existing reaginic antibody formation nor suppress secondary antibody response.