Lymphocyte differentiation and specific antibody synthesis were studied in the course of the secondary immune response of horseradish peroxidase immunized rat popliteal lymph nodes by electron microscopic immunocytochemistry. From primary immunization, mature plasma cells containing specific antibody remained as long as 10 months. Antihorseradish peroxidase antibodies were localized in lymphocytic blast cells on Day 3 after restimulation with antigen. In the subsequent days blast cells were still present, and shifting to mature stages occured. During immunocyte differentiation and maturation vesicles and tubules of the Golgi complex became filled with specific antibody suggesting occasional secretion of antibody via the Golgi apparatus. At very mature cell stages, antibody was accumulated in distended cisternae.