Substantial evidence has accumulated recently that in cell membranes composition fluctuations and microdomains exist, also referred to as rafts. The structure of ternary model raft lipid mixtures, such as sphingomyelin, cholesterol and DOPC (1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) or POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine), was studied by small-angle neutron scattering using the H/D contrast variation technique. The lipid mixtures exhibit a Q⁻³ power-law scattering, indicating the existence of composition fluctuations, which correspond to space-filling droplets with a broad size-distribution ranging from about 20 to 200 nm. No indications of critical-like concentration fluctuations are visible in the concentration and temperature range (15–60 °C) covered. The data clearly demonstrate the existence of microdomains in the model raft mixtures not only in the μm range as revealed by fluorescence microscopy, but also their existence over a large length scale down to the range of several nanometres. In addition, we were able to directly visualize the resistance of part of the lipid domains against non-ionic detergent (Triton X-100) in situ.