The activity of granzyme B, a main effector molecule of cytotoxic T lymphocytes (CTL) and natural killer cells, is regulated by the human intracellular serpin proteinase inhibitor 9 (PI9). This inhibitor is particularly expressed by CTL and dendritic cells, in which it serves to protect these cells against endogenous and locally released granzyme B. Moreover, PI9 expression by neoplastic cells may constitute one of the mechanisms for tumors to escape immune surveillance. Here we show that PI9 is also expressed by human mast cells. In immunohistochemical studies using a PI9-specific monoclonal antibody, strong cytoplasmic staining for PI9 was found in normal mast cells in various tissues throughout the body. In addition, in 80% of all cases of cutaneous and systemic mastocytosis tested the majority of the mast cells expressed PI9. As an in vitro model for PI9 expression by mast cells, we studied expression by the human mast cell line HMC-1. Stimulation of HMC-1 with PMA and the calcium ionophore A23187 resulted in a marked increase of PI9 expression. Thus, PI9 is expressed by activated mast cells. We suggest that this expression serves to protect these cells against apoptosis induced by granzyme B released during initiation of the local inflammatory response.