The degranulation of mast cells under the influence of compound 48/80, polymyxin B sulphate and specific antigen was studied by scanning electron microscopy. It was observed that granules appeared to leave the cell through a process of exocytosis. In addition, pores appeared on the surface of treated cells. No differences were noted in the morphology of degranulation brought about by the three agents tested. It is concluded that both exocytosis and pore formation are significant mechanisms in mast cell degranulation.