A feeder layer independent long-term in vitro culture system for murine mast cells is described. Concanavalin A-activated murine splenic leukocyte-conditioned medium, prepared under conditions optimal for T cell growth factor production, has been found also to contain a growth-promotion activity for murine mast cells identified by their morphology, characteristic ultrastructure of the granules, positive reactions with toluidine blue and alcian blue, presence of receptors for IgG and IgE, as well as presence of histamine, serotonin, L-Dopa, 5-hydroxytryptophan, and sulfated products within the cytoplasm. After 2 to 3 wk of culture in the presence of the conditioned medium, mast cell lines were established from various sources initially devoid of matured mast cells. Such sources included spleen and bone marrow of athymic nude mice, long-term cultured marrow cells as well as T cell-depleted normal marrow. Cultured mast cells are absolutely dependent upon the conditioned medium-derived growth factor(s) for growth and viability. Death ensues within 24 hr in the absence of the factor(s). Established mast cell lines have been maintained in exponential growth for over 1 yr by passaging in the conditioned medium every 3 to 7 days.