Disodium cromoglycate binds in vitro and in vivo to lipids in white cells. Smears of cells from lymphocyte cultures and from bone marrow aspirates treated with DNCG and subsequently stained with pseudoisocyanine show a characteristic green fluorescence (515 nm) of membrane- and intracellular-lipids. It is suggested that the mode of action of DNCG in the prophylaxis of bronchial asthma could be the binding of DNCG to membrane lipids. This binding might block the IgE-mediated reaction on the surface of mast cells which otherwise would lead to degranulation and release of vasoactive substances.