Mice were immunized with horseradish peroxidase and injected with [3H]thymidine. Popliteal lymph node cells were submitted to electron microscopic immunocytochemistry and high resolution autoadiography in order to correlate antibody production and ability to undergo cell division at various stages of lymphoid cell differentiation. Antibody synthesis started in blast cells and increased steadily until the mature plasma cell stage was reached. Thymidine incor-poration was highest in blastoid cells and decreased continuously afterwards. Chromatin dispersion was found to be paralleled by thymidine incorporation. Almost all mature plasma cells were devoid of thymidine incorporation. This confirms that they are end cells apparently unable to divide.