Lipid rafts - ordered membrane microdomains enriched in sphingolipids,cholesterol and GPI-anchored proteins - play important roles in membrane trafficking and signalling. In mast cells, for example, signalling through the IgE receptor FcϵRI appears to require coalescence of lipid rafts, which allows the receptor to switch on the Src-family kinase Lyn. Barbara Baird and co-workers have now developed a new tool for analysis of raft function:short-chain ceramides, membrane-permeant molecules that are often used to mimic the naturally occurring long-chain ceramides that cells generate by hydrolysing sphingomyelin. The authors show that C2-ceramide and C6-ceramide disrupt lipid rafts: they decrease fluorescence anisotropy (a measurement of membrane order) in membrane vesicles and reduce fluorescence resonance energy transfer (FRET) between raft-associated molecules in intact cells (see p. 3177). Baird and co-workers then use these novel inhibitors to dissect IgE/FcϵR signalling, demonstrating that they cause similar inhibition of FcϵRI-induced Ca2+ mobilization and phospholipase D (PLD)activation. Finally, they show that the PLD inhibitor n-butanol also blocks both Ca2+ mobilization and PLD. Their findings thus not only reveal a new raft inhibitor, but also place PLD upstream of antigen-stimulated Ca2+ mobilization in the IgE/FcϵR signalling pathway.