The origins of immunoglobulin-containing cells in intestinal, respiratory, mammary, and genital tissues were studied in CBA/J female mice by using an adoptive lymphocyte transfer method. Within 24 hr after transfer, [³H]thymidine-labeled donor mesenteric lymph node (MLN) cells were observed in recipient gut, cervix and vagina, uterus, mammary glands, and MLN, where approximately 60% contained IgA and 25% IgG. In peripheral lymph nodes (PLN), 44% of the labeled cells after MLN transfer contained IgG, whereas only 8% were of the IgA isotype. The preference of the MLN to populate mucosal sites was clear from the results. Labeled PLN cells were transferred and the majority of these returned to their sites of origin and contained IgG. Of the small number of labeled PLN cells found in mucosal tissues, approximately equal percentages (30%) of IgA- and IgG-containing cells were seen. Dividing cells prepared from mediastinal (bronchial) lymph nodes (BLN) showed a propensity to localize in the lungs rather than the intestine. However, the predominant immunoglobulin content of these donor cells in gut, lungs, and MLN was IgA. In recipient PLN, most labeled BLN cells contained IgG. These data support the concept of a common mucosal immunologic system.