1. The low-density lipoproteins (LDL; density 1·007–1·063 g/ml) from two patients homozygous for familial hyperbetalipoproteinaemia have been submitted to chemical and physicochemical analysis.
2. The presence of an anomalous lipoprotein with a low proportion of triglyceride and a raised proportion of cholesterol has been confirmed.
3. In one patient, this lipoprotein accounted for about 85% of the LDL, but in the second, the amount varied from about 85% to a point at which it could not be detected among the coexisting normal lipoproteins.
4. The protein moiety of this anomalous LDL has effectively the same amino acid composition as that derived from the LDL of healthy subjects.
5. The proportions of carbohydrate, phospholipid and fatty acids could not be reliably distinguished from those of normal LDL.
6. The molecular weight and diffusion constant of the abnormal lipoprotein, even in the purest preparation, were close to the values determined for normal LDL of similar flotation rate.