A study has been made of the relationship of antibody formation and the changes in amount of the nucleic acids in rabbit lymph nodes draining areas injected with typhoid vaccine. The increase in DNA was found to parallel the increase in weight of the nodes, as might be expected from the active multiplication of cells. The peak of PNA increase occurred between the 4th and 6th days after vaccine injection when antibody formation was at its maximum. A histologic study of methyl green- and pyronine-stained sections of the nodes revealed that during the first 6 days of the experiment the cellular reaction was chiefly one of plasma cells. During the first 3 days plasmoblasts predominated; on the 5th and 6th days mature plasma cells were the prevailing cells. Most of the PNA was contained in the plasma cells. The lymphocytes began to proliferate in significant numbers on the 3rd and 4th days, and germinal centers began to appear on the 4th and 5th days. They showed their greatest activity only on the 9th day when PNA and antibody formation had passed their peaks. These results are interpreted as indicating that the plasma cell and not the lymphocyte is responsible for antibody formation.