Mouse mast cells were differentiated and grown by culturing bone marrow cells in medium containing 2 X 10(-10) M purified interleukin 3 (IL 3). The cells obtained were similar in ultrastructure, membrane antigen phenotype, proteoglycan type, and lipid products generated upon immunologic activation to mast cells differentiated in culture by WEHI-3-conditioned medium (WEHI-3-CM) and by concanavalin A (Con A) splenocyte-conditioned medium. Phenotypically, these cells expressed IgE receptors and H-2 antigens and were recognized by a monoclonal antibody (B23.1) that did not react with mouse serosal heparin-containing mast cells. The classic phenotypic markers of mouse T cells or macrophages were not detected. The mouse mast cells differentiated with IL 3 as well as those differentiated in WEHI-3-CM incorporated [35S]sulfate into a nonheparin proteoglycan of 150,000 to 200,000 m.w. Most of the 35S-labeled macromolecules were degraded by chondroitinase ABC to yield only two disaccharides, which co-chromatographed on ascending thin layer chromatography with delta Di-4S and delta Di-diSE; thus, the proteoglycan in these cells is composed of chondroitin sulfate E glycosaminoglycans. After sensitization with monoclonal IgE, washing, and antigen activation, the IL 3 differentiated cells released the preformed mediator beta-hexosaminidase and generated and released two major classes of lipid mediators. The quantities of leukotriene C4 (LTC4), leukotriene B4 (LTB4), and platelet-activating factor (PAF-acether) generated/10(6) cells were 17, 3.0, and 3.1 ng, respectively. The ratio of these three lipid mediators was similar to that obtained from mast cells differentiated in WEHI-3-CM and in Con A-conditioned medium. Thus, T cell-derived IL 3 is the component present in the conditioned media that is required for differentiation and growth of the subclass of mast cells containing chondroitin sulfate E proteoglycan, designated E-MC. The IL 3-dependent E-MC may represent the in vitro counterpart of the T-cell-dependent mucosal mast cell, suggesting in turn that the production of LTC4 and LTB4 and of PAF-acether may play a role in adaptive intestinal immunity to helminthic parasites.