Abstract In rabbits given complete Freund's adjuvant or complete adjuvant and diphtheria toxoid there were two types of cells in the spleen which contained antibody and gamma globulin. Non-phagocytic mononuclear cells containing 19S γ 1M antibody and gamma globulin were associated with the appearance and level of circulating 19S γ 1M globulin antibody. Gamma globulin and 7S γ 2 antibody in the cytoplasm of the plasma cells were associated with the appearance and level of circulating 7S γ 2 globulin antibody. Changes in the basophilia and ultrastructure of the cytoplasm of these cells were consistent with increased protein synthesis. The mononuclear cell was present in the red pulp, and the development of the plasma cell occurred primarily in the non-follicular white pulp.