Summary: The qualitative aspects of the 7S antibody response in the mouse have been studied using radio-immunoelectrophoresis. The production of a fast gamma (7Sγ1) and/or a slow gamma (7Sγ2) antibody was influenced by four separate factors: the antigen, the adjuvant, the strain of mouse and the previous exposure of the mouse to the antigen. A sequence of 7S antibody production was detected in certain circumstances and characterized by an initial antibody consisting of fast γ (7Sγ1)-, and a later antibody of slow γ (7Sγ2)-globulin.