Abstract Low-density lipoprotein from avian yolk has been studied by electron microscopy, and subfractionated after repeated freezing and thawing. The fractions were analysed chemically and examined by electron microscopy and X-ray diffraction. Results indicate that: 1. (i)|The surface layer of LDF is probably heterogeneous and that freezing and thawing liberates a component part of the surface layer. 2. (ii)|The surface component contains mainly phospholipid, neutral glycerides, and protein, with some unesterified cholesterol. 3. (iii)|The lipid and protein components do not form a stable association of the kind demonstrated in a number of biological membranes.