The data presented in this paper support a common source for intestinal mast cells in intestinal mucosae. IMC precursors and/or mastocytopoietic factors of as yet unknown origin could be transferred by immune mesenteric lymph node (IMLN) cells of thymus-bearing mice. The differentiated locally or induced mastocytopoiesis in Trichinella spiralis infected mucosal sites, but only in animals with a normal thymus function. Antigen-stimulated precursor cells either in the gut or in the mesenteric lymph node to enter the circulation and home to both parasitized and nonparasitized mucosal sites. Both proliferation in the mesenteric lymph node and in the intestinal mucosae were shown to be thymus-dependent. The injected IMLN stimulated only the proliferation of IMC and not of globule leucocytes. This might indicate that cells relevant for IMC proliferation are different from those essential for GL proliferation.