Paired Ig-like receptors (PIR) are polymorphic type I transmembrane proteins belonging to an Ig superfamily encoded by multiple isotypic genes. They are expressed on immune cells such as mast cells, macrophages, and B lymphocytes. Two subtypes of PIR have been classified according to the difference in the primary structure of the PIR transmembrane and cytoplasmic regions. These subtypes are designated as PIR-A and PIR-B. In this study, the transmembrane and cytoplasmic regions of the PIR-A subtype were shown to mediate activation signal events such as cytoplasmic calcium mobilization, protein tyrosine phosphorylations, and degranulation in rat mast cell line RBL-2H3. The association of the Fc receptor γ and β subunits with PIR-A was shown to be responsible for PIR-A function but not required for membrane expression of PIR-A on COS-7 cells. We further revealed the role of two charged amino acid residues in the transmembrane region, namely arginine and glutamic acid, in PIR-A function and its association with the above subunits. In contrast to the inhibitory nature of the PIR-B subtype, present findings reveal that PIR-A potentially acts as a stimulatory receptor in mast cells, suggesting a mechanism for regulation of mast cell functions by the PIR family.