This paper describes a method for the specific histochemical demonstration of antibody, which was used to study the response to antigenic stimulation. The method involves a two-step immunological reaction on frozen tissue sections: first, allowing antibody in the tissue to react with dilute antigen applied in vitro, and second, detecting areas where this antigen has been specifically absorbed using a precipitin reaction with fluorescein-labeled antibody. Under fluorescence microscopy, the yellow-green fluorescence of fluorescein reveals the areas where a precipitate has formed, indicating the presence of antibody. The study of hyperimmune rabbits showed that antibody against human γ-globulin or ovalbumin is present in groups of plasma cells in the red pulp of the spleen, the medullary areas of lymph nodes, the submucosa of the ileum, and the portal connective tissue of the liver. However, due to extensive non-specific reactions, the bone marrow could not be examined. Small amounts of antibody were occasionally visible in cells in the lymphoid follicles of the spleen and lymph nodes, suggesting a minor contribution by lymphocytes to antibody synthesis. The method is specific despite the occurrence of non-specific reactions. It relies on the use of two layers, with antigen fixed to the section in vitro serving as a bridge between the antibody in the tissue and the labeled antibody used as a histochemical reagent. The results indicate that the major site of antibody formation is a family of cells that first appear as a response to the stimulus. The mature member of this cell family is the plasma cell. The study also found that the spleen contains large numbers of plasma cells, which are the main site of antibody production. These cells contain high concentrations of antibody, and their presence correlates with active in vitro synthesis. The absence of antibody in detectable concentrations in hepatic cells or Kupffer cells is consistent with findings that the perfused rat liver fails to incorporate C¹⁴-labeled lysine into γ-globulin, and that there is no antibody production in vitro by the liver of rabbits that had received several injections of bacillary antigens. The observation of small but definite incorporation of C¹⁴-labeled glycine into antibody by the liver of hyperimmune rabbits is explained by the presence of plasma cells in the portal connective tissue and the observation that such cells in this location contain antibody.This paper describes a method for the specific histochemical demonstration of antibody, which was used to study the response to antigenic stimulation. The method involves a two-step immunological reaction on frozen tissue sections: first, allowing antibody in the tissue to react with dilute antigen applied in vitro, and second, detecting areas where this antigen has been specifically absorbed using a precipitin reaction with fluorescein-labeled antibody. Under fluorescence microscopy, the yellow-green fluorescence of fluorescein reveals the areas where a precipitate has formed, indicating the presence of antibody. The study of hyperimmune rabbits showed that antibody against human γ-globulin or ovalbumin is present in groups of plasma cells in the red pulp of the spleen, the medullary areas of lymph nodes, the submucosa of the ileum, and the portal connective tissue of the liver. However, due to extensive non-specific reactions, the bone marrow could not be examined. Small amounts of antibody were occasionally visible in cells in the lymphoid follicles of the spleen and lymph nodes, suggesting a minor contribution by lymphocytes to antibody synthesis. The method is specific despite the occurrence of non-specific reactions. It relies on the use of two layers, with antigen fixed to the section in vitro serving as a bridge between the antibody in the tissue and the labeled antibody used as a histochemical reagent. The results indicate that the major site of antibody formation is a family of cells that first appear as a response to the stimulus. The mature member of this cell family is the plasma cell. The study also found that the spleen contains large numbers of plasma cells, which are the main site of antibody production. These cells contain high concentrations of antibody, and their presence correlates with active in vitro synthesis. The absence of antibody in detectable concentrations in hepatic cells or Kupffer cells is consistent with findings that the perfused rat liver fails to incorporate C¹⁴-labeled lysine into γ-globulin, and that there is no antibody production in vitro by the liver of rabbits that had received several injections of bacillary antigens. The observation of small but definite incorporation of C¹⁴-labeled glycine into antibody by the liver of hyperimmune rabbits is explained by the presence of plasma cells in the portal connective tissue and the observation that such cells in this location contain antibody.