A restriction fragment length polymorphism (RFLP) at the low density lipoprotein receptor (LDLR) locus, detectable with the restriction enzyme PvuII has been studied in a second series of Norwegian subjects, believed to be representative of the general population. The results confirm the association of normal alleles at the LDLR locus with differences in age- and sex-corrected total and LDL cholesterol. A gene identified as one of the alleles in this RFLP appears to eliminate the effect of the apolipoprotein E4 (apoE4) gene on cholesterol (or its allele must be present for the apoE4 effect on lipid level to manifest itself). The findings in this series substantiate previous indications that normal alleles are of importance in the control of LDLR activity and that normal LDLR alleles contribute to the population variation in cholesterol. Finally, they confirm that an interaction between LDLR and apoE genes contributes to the population variation in total and LDL cholesterol.