Mast cells were obtained from mouse bone marrow cells cultured for 14 days in medium derived from Concanavalin A (Con A) stimulated mouse spleen cells. Upon passive sensitization of the cultured cells with immunoglobulin E (IgE), histamine release from mast cells was approximately 200% above control within 1 min of incubation with anti-IgE. The calcium inonophore A 23187 also evoked a concentration-dependent (10(-8) M to 6 x 10(-7) M) histamine release following a 6 min incubation. Transmission electron microscopy (TEM) demonstrated that the secretory granules of the cultured cells have a peripheral crystalline pattern, like that previously demonstrated for mast cells. Scanning electron microscopy (SEM) illustrated spherical cells with surfaces traversed by many ridge-like folds. Intragranular fusion, following exposure of the IgE-bearing cells to anti-IgE, led to accumulation of the granules into channels which, on study with both transmission and scanning electron microscopy, appeared to be associated with the cell surface.