1. The concordance of low density lipoprotein (LDL) receptor activity of blood mononuclear cells was examined in 26 pairs of monozygotic and 17 pairs of dizygotic normolipidaemic young adult male twins.

2. Receptor activity was quantified as the degradation of 125I-labelled LDL during a 6 h incubation, after derepression of the cells for 72 h in lipoprotein-deficient medium.

3. The total variance of receptor activity was similar in the two groups of twins. In contrast, within-pair variance was five times greater in dizygotic twins than in monozygotic twins (P < 0.001).

4. Estimates of heritability, mostly based on the intra-class correlation coefficients (monozygotic, r = 0.83; dizygotic, r = 0.39), ranged from 0.72 to 1.05.

5. These results suggest that the maximal LDL receptor activity of peripheral cells in normolipidaemic subjects is largely genetically determined.

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