1. Plasma low-density lipoprotein (LDL) from a patient with homozygous familial hyperbetalipoproteinaemia was labelled with 125I and LDL from a matched normal subject was labelled with 131I. The two samples of LDL were injected simultaneously into the patient and into a second normal subject.
2. In each subject the fractional rate of turnover of the two LDL samples was the same, but the fractional rate in the patient was less than half that in the normal subject.
3. It is concluded that the hyperbetalipoproteinaemia in the patient was not due to an abnormality in her LDL protein recognizable by her own body or by that of the normal recipient.