Autologous 131I-labelled very low density lipoprotein (VLDL) and 125I-labelled low density lipoprotein (LDL) were injected into seven normal subjects and twenty-eight genetically classified hyperlipidaemic patients to quantitate lipoprotein interconversion. The apoprotein B specific activity--time curves for VLDl and intermediate density lipoprotein (IDL, density = 1 . 006--1 . 019 g/ml) intersected at or before the IDL-B maximum in thirty-one studies (five normal controls and twenty-six hyperlipidaemic subjects) implying that all IDL-B may be derived from VLDL-B. The fractional conversion of VLDL-B to LDL-B (density 1 . 019--1 . 063 g/ml) following a simultaneous spike injection of 131I-VLDL and 125-LDL was obtained by deconvolution of the 125I and 131I-LDL-B activity curves. 21--65% (mean = 44%) of VLDL-B was converted to LDL-B in twenty-three subjects studied. The mean conversion time ranged from 10 to 24 h in ten normotriglyceridaemic subjects and from 19 to 42 h (mean = 33 h) in twelve hypertriglyceridaemic subjects. In one patient with broad-beta disease the mean conversion time was 55 h. LDL-B production from VLDL-B and total LDL-B synthetic rate were essentially equal in normal controls and normocholesterolaemic subjects and in the patient with broad-beta disease. But in all six patients with familial hypercholesterolaemia LDL-B synthetic rate significantly exceeded LDL-B production from VLDL-B, indicating direct secretion of 20--72% of LDL-B at a rate which correlates positively with plasma LDL concentration. Three of five patients with familial combined hyperlipidaemia showed a lesser but nevertheless significant direct secretion of LDL-B.