The present study was carried out in vitro to determine whether murine Peyer's patches (PP) can be a source of mucosal mast cells, using a well-defined mast cell growth factor, interleukin 3 (IL-3). The non-T, non-B, non-adherent (null) cell population of PP contained the precursor cells for mast cells. The mast cells induced possessed toluidine blue and alcian blue (pH 3)-positive granules, which were heterogeneous and contained histamine. The IL-3-induced mast cells were IL-3-dependent, did not express Ia, T-cell, B-cell or macrophage markers, and released histamine in response to Ca ionophore, but not in response to compound 48/80. These cells bore IgE-specific surface receptors and intracellular 20 alpha hydroxysteroid dehydrogenase. The latter enzyme activity was induced by IL-3 in null cells. All these features of the mast cells support the view that these cells belong to a lineage of mucosal mast cells (MMC) and are no different from those of other lymphoid tissues, such as the bone marrow and spleen. In the analysis of MMC precursor frequency, PP had a similar frequency to that of other gut-associated lymphoid tissues (mesenteric lymph nodes and lamina propria), but was much lower when compared to those of the bone marrow and spleen. Furthermore, like IL-2, PP are capable of producing IL-3 in vitro by activating their helper marker-bearing T cells. Thus, under certain conditions, such as the initial participation by PP in MMC-associated intestinal immune responses, PP are likely to provide MMC in their microenvironment, like mucosal T and B cells.