The distribution of background Ig-secreting cells, measured as cells containing cytoplasmic immunoglobulin (C-Ig cells), over spleen, bone marrow, lymph nodes and Peyer's patches was studied in congenitally athymic (nude) mice and heterozygous euthymic mice as a function of age and immune status (germ-free (GF) vs specific pathogen-free (SPF]. In young athymic as well as in young euthymic mice, the spleen was found to contain the great part of all C-Ig cells, irrespective of whether the mice were GF or SPF. The number of C-Ig cells in the spleen was found to be rather constant over the life span, while the number of C-Ig cells in the bone marrow of all groups of mice greatly increased with age. This indicates that the relative shift of C-Ig cells to the bone marrow is neither dependent on the presence of the thymus, nor on the microbiological status of the mice. However, at young and intermediate age the microbiological status of the mice did affect the total number of C-Ig cells per mouse. This was mainly due to the effect upon the bone marrow, mesenteric lymph nodes and Peyer's patches. At these ages the background Ig synthesis in these organs appeared to be mainly dependent on external antigenic stimulation, in contrast to the spleen, where the Ig synthesis appeared to be mainly due to endogenous stimulation. The Ig (sub)class distribution of the C-Ig cells was different for all different organs tested. Hardly or no difference in percentage distribution was found between the GF nude and GF heterozygous mice. Most C-Ig cells in spleen, bone marrow and lymph nodes of the GF mice were of the IgM isotype. C-IgG and C-IgA cells occurred in substantial percentages only in bone marrow and lymph nodes. In the lymph nodes of GF nude mice a remarkably high percentage of C-IgA cells was found.