Cloned, immature mast cells derived from normal mice were passively sensitized with mouse monoclonal IgE antibodies with specificity for DNP, and then stimulated to degranulate with DNP35-HSA. Cells were fixed for transmission electron microscopy or recovered for quantitation of histamine release at various intervals up to 30 minutes after antigen challenge. The cloned mast cells rapidly extruded the contents of their immature granules (dense progranular material and membrane-bound vesicles) to the exterior via multiple openings in the plasma membrane. Degranulation was associated with striking activation of the cell surface, characterized initially by elongation of surface processes, as well as by close approximation of strands of rough endoplasmic reticulum to the cell surface and by the development of coated pits. At later times after stimulation, degranulated mast cells had released nearly all of their granules and exhibited angular surfaces lacking elongated processes. These findings demonstrate for the first time that cloned, immature mast cells, like their mature counterparts, can undergo classic morphologic release reactions involving exocytosis of granules.