Granules of mast cells contain chemical mediators, histamine and SRS-A, and degranulation of the cell is induced by immunological reaction. To identify a relation between immunological reaction mediated by IgE or IgG and surface ultrastructures of mast cells, a probe was constructed for immunoglobulin receptors on the cell surface by covalently coupling antigen to latex particles (0.455±0.010μm in diameter). Histamine release from mast cells and morphological aspects were examined using a technique for immune-scanning electron microscopy, when antigen was incubated with cells which were passively sensitized with antiserum. The following results were obtained: 1) IgE receptors were detected by the binding of numerous latex particles on the surface of the mast cell, in contrast with control experiments for passive sensitization with or without heat-inactivated anti-serum. Mast cells which showed ridge-and ruffle-dominant type or raspberry type, were ultrastructurally recognized in the immunological reaction, though control cells were of the microvilli-dominant type. 2) IgG-mediated reaction was recognized on the mast cell by marked release of histamine from cells and by a low degree of staining with toluidine blue. Surface ultrastructure of mast cells was also changed as those of the IgE-mediated reaction. Moreover, degranulation of mast cells was observed at the site of patch or cap formation of the receptor. In conclusion, these findings indicate that rat mast cells have both IgE and IgG receptors. Moreover, mast cells are activated and are induced to release these granules by the immunoglobulin-mediated reaction.