Summary Stimulation of guinea pigs with protein antigen incorporated in complete Freund's adjuvant results in sequential production of three types of immunoglobulins. The initial response of rapidly sedimenting or macroglobulin antibody is superseded by a γ1 (PCA) 7 S antibody response. Subsequently the γ2 (cytotoxic) 7 S antibody appears in significant amounts. This last stage signals full maturation of the circulating antibody response so far as it can be measured by current techniques. By extirpation of the stimulated regional lymph nodes at discrete intervals during the differentiation process and injection of the dispersed cells into isologous normal recipients, the various stages of the primary immune response can be isolated and studied. In the absence of additional antigen, the types of antibody detectable in the donor at the time of transfer are the only kinds produced in the recipient. The possible mechanism of this arrest of maturation and the application of this model system are discussed.