Antibodies against haemopoietic differentiation antigens are widely used for the identification and classification of leukaemic cells. Mast cells are distinct haemopoietic cells and express a unique composition of antigens. They can be differentiated from basophils and all other lymphohaemopoietic cells by using antibodies to cell surface receptors and granular mediator molecules. This paper provides evidence that antibodies against mast cell antigens can be used to identify and classify malignant cell populations in patients with mastocytosis.