The number of cells containing cytoplasmic immunoglobulin (C-Ig cells) was determined in the spleen, mesenteric lymph nodes, bone marrow and Peyer's patches of CBA mice of different ages. A rapid increase in the number of C-Ig cells at between 2 and 6 weeks of age was observed in spleen and gut-associated lymphoid organs. The absolute number of C-Ig cells in these organs decreases with advancing age. In the bone marrow, the number of C-Ig cells increases steadily with age up to one year. From one year on, the number remains approximately constant in the males. In female mice, the number of C-Ig cells, mainly of the IgA class, increases sharply around 1 year of age. The spleen is the major site of Ig synthesis up to about 6 months of age. In older animals, the relative contribution of the bone marrow increases with age, possibly due to a gradual shift in the individual animal from primary type responses to a pattern of secondary type responses. No indication of a decreased overall immunological activity in senescence was obtained.