Mast cells are hemopoietic effector cells producing diverse mediators.They are present in low numbers in the bone marrow but increase insome pathological states as preleukemic syndromes,lymphoproliferativedisorders & mastocytosis.In this work, we used antitryptase antibody, AA1, in detection of bonemarrow mast cells in different pathological states. This methods showhigh sensitivity & specificity & offers distinct advantages over manyother techniqes currently in use for mast cell detection.