We report on the formation of bicelles from a mixture of dimyristoylphosphatidylcholine (1)M PC) and the chelator-lipid dimyristoylphosphatidylethanolamine-diethylenetriaminepentaacetate (DMPE-DTPA) with complexed lanthanides, either thulium (Tm3+) or lanthanum (La3+). The two phospholipids used have the same acyl-chain length but differ in headgroup size and chemical structure. The total lipid concentration was 15 mM, and the molar ratio of DMPC to DMPE-DTPA was 4:1, The system was studied with small angle neutron scattering (SANS) in a magnetic Field, cryo-transmission electron microscopy (cryo-TEM), and P-31 NMR spectroscopy. We found that, after appropriate preparation steps, that is. extrusion through a polycarbonate membrane followed by a cooling step, monodisperse small unilamellar disks (flat cylinders called bicelles) are formed. They have a radius of 20 urn and a bilayer thickness of about 4 nm and are stable in the investigated temperature range of 2.5-30 degrees C. Fitting of SANS data with a form factor for partly aligned flat cylinders shows that the bicelles are slightly orientable in a magnetic field of 8 T if DM PE-DTPA is complexed with Tm3+.