Lymphoid and bone marrow cells from normal and horse serum-immunized mice and lymphoid cells from Nippostrongylus brasiliensis-infected mice were cultured on monolayers of embryonic skin fibroblasts to analyze the factors which regulate the differentiation and proliferation of mast cells in vitro. Our results indicate that T cells can regulate the development of mast cells in vitro by either enhancement or suppression. In cultures of horse serum-immune spleen cells, inducer T cells are required for mast cells to develop. However, in cultures of mesenteric lymph node cells from N. brasiliensis-infected mice, inducer T cells are not required for mast cell development. This suggests that the development of mast cells may occur at discrete interleukin-3 (IL-3)-dependent and IL-3-independent stages. Mast cell precursors in the mesenteric lymph nodes of N. brasiliensis-infected mice may have already been acted on by inducer cells in vivo to become mast cell committed. While IL-3 does not appear to be required for mast cell development, these precursors do require the presence of a connective tissue microenvironment such as embryonic skin. The precursors can be inhibited from development by interferon preparations.