In this study we investigated the contribution of the small intestine of C3H/He mice to the spontaneous ('background') immunoglobulin (Ig) production in terms of the number of Ig-secreting cells (Ig-SC), and compared the results with the numbers of Ig-SC found in various lymphoid organs (spleen, bone marrow, mesenteric lymph nodes and Peyer's patches). The results show that in C3H/He mice of 20 weeks of age, on average 16 x 10(6) Ig-SC can be isolated from the small intestine. Almost all of these Ig-SC produce IgA. Compared to the other lymphoid organs, the small intestine contains more than 80% of all Ig-SC present in adult C3H/He mice. These results are in agreement with the need to maintain relatively high levels of Ig at the mucosal surfaces, especially of secretory IgA, for the prevention of penetration of these surfaces by micro-organisms. In man and mouse most of these Ig are supposed to be produced locally in the underlying mucosal tissues and subsequently transported across the epithelium. Although the IgM and IgG levels in serum are predominantly maintained by non-mucosae associated lymphoid organs, the results of this study clearly indicate that the mucosal tissues are the major site of 'background' Ig-production.