IgA secretion by the lactating mammary gland culminates a complex sequence of biologic events both within the gland and at distant sites. Because of the many extraglandular influences, it is difficult to investigate IgA secretion at the tissue and cellular levels in the intact animal. In this study, with the use of immunohistofluorescence, we have observed elaboration of IgA by primary monolayer cultures of mammary cells from the glands of mid-pregnant mice and from mammary tumors. In cultures of normal cells, IgA appeared first in vesicular structures on the upper surfaces of the monolayers. Vesicular IgA was maximal at day 5 in culture, and at that time, was observed only over mounds (domelike structures) that were covered with fibrillar fibronectin (FN), and eventually developed a subpopulation of fusiform cells. It appears that the IgA observed was secreted in vitro, that normal mammary epithelial cells must form multicellular FN-bearing structures to secrete IgA in culture, and that by mid-pregnancy the murine mammary gland contains all of the lymphoid cells required for IgA secretion. In contrast, primary cultures of mammary tumors displayed minimal amounts of IgA and FN. The small amount of cell-associated IgA that was detected on tumor cultures was not localized to any multicellular structures nor was it associated with FN, but instead appeared as minute, punctate accumulations on individual cells scattered across the epithelioid areas. This finding is consistent with the loss of specialized functions and the increased autonomy typical of malignant cells. The study in cultured cells of a function as specialized as IgA secretion should permit greater understanding of both the process itself and the despecializations that accompany malignancies of secretory epithelia.