Abstract Mast cells hold a key position in the defensive mechanisms against exogenous intruders. In this study, we investigated whether human mast cells express functional major histocompatibility complex (MHC) class II molecules that can transduce endogenous signals and present staphylococcal enterotoxin A (SEA) to T cells. Similar to HMC-1 human mast cell line, umbilical cord blood-derived mast cells express HLA-DR, -DP and -DQ molecules on their surface. MHC class II molecules expressed on HMC-1 cells bind significantly the SEA (a natural MHC class II ligand), and their ligation with specific mAbs or with SEA, leads ultrastructural chages, suggesting their degranulation. Recognition of SEA-bound MHC class II molecules on HMC-1 mast cells by the T cell receptor of K25 cells, an SEA-specific murine T cell hybridoma, triggers significant IL-2 secretion by these T cell hybridomas. Hence, our data point out the expression of functional MHC class II molecules on human mast cells, reinforcing the implication of these cells in the defense mechanisms of acquired immunity. J. Leukoc. Biol. 64: 791–799; 1998.