After primary immunization of mice, rats and rabbits with antigens (horse radish peroxidase, bovine serum albumin and muchroom tyrosinase) emulsified in complete or incomplete Freund's adjuvant, both cells synthesizing immunoglobulin without detectable antibody function and antibody-producing cells were detected. The first cells which appeared were synthesizing and secreting IgG and IgM immunoglobulins without antibody function. These cells were progressively replaced by cells synthesizing and secreting antibodies. In some plasma cells of mice, rats and rabbits immunized with peroxidase, antibody activity was detected only in restricted areas of the cytoplasm ; the remainder contained antigenic determinants of immunoglobulins. After secondary immunization the results were the following: in mice, both cells containing immunoglobulins without antibody function and antibody-containing cells appeared simultaneously and they were present in equal amount; in rats, only the antibody-containing cells were present in high number. Immunizations performed using different protein antigens (horse radish peroxidase, human and bovine serum albumin, aggregated and desaggregated human IgG, and ovalbumin) injected as a solution in saline have shown that after antigenic stimulation both populations of cells appeared, their number depending on the dose of the antigen injected. Further experiments carried out with tolerant mice, with germ-free animals and with "B" mice have shown that the appearance or not of antibody-producing cells was always related with respectively the presence or absence of cells synthesizing immunoglobulins without detectable antibody function. Finally experiments performed on rabbits have shown that some cells containing immunoglobulins without antibody function share idiotypic determinants in common with cells synthesizing antibodies.