It is difficult to isolate and impossible to propagate human mast cells in tissue culture. As an alternative to the use of human differentiated mast cells, a human leukaemic mast cell line (HMC-1), which can be propagated in vitro, has been employed in a number of studies. Carbohydrate binding proteins, lectins, have been used to characterise the terminal sugar residues of human mast cells in situ. The aim of the present study is to characterise the lectin binding sites of HMC-1 cells transplanted into severe combined immunodeficient (scid) mice. Lectins specific for the complex carbohydrates, neuraminic acid and N-acetylglucosamine residues showed generally a strong uniform binding pattern, whereas mannose and glucose specific yielded lectins a greater heterogeneity. This glycotope expression pattern has some similarities with those of human mast cells in situ, and therefore HMC-1 cells grown in scid mice constitute a valuable model system for the study of carbohydrate expression in human mast cells.