Mast cells and basophils are implicated as major effector cells in allergic disease. However, both mast cell and basophil involvement in clinical events have been difficult to assess heretofore because of localization of mast cells in tissues and the small numbers of basophils in the circulatory system. Tryptase has been found to be a discriminating marker for the participation of human mast cells in immediate allergic responses, and therefore provides precise assessment of mast cell activation. High tryptase levels in serum, plasma, and other biologic fluids are consistent with mast cell activation in systemic anaphylaxis and other immediate hypersensitivity allergic reactions. Although basophil activation has been implicated in late phase response to allergen challenge, sensitive specific indicators of basophil activation are still under investigation.