An epitope of Apolipoprotein B (ApoB), recognised by a monoclonal antibody BIP-45, is associated with the development of ischaemic heart disease (Duriez et al. 1988). We have examined the genetic relationships between this epitope and three Restriction Fragment Length Polymorphisms (RFLPs) of the gene for ApoB detected with the enzymes EcoRI, PvuII and XbaI in a sample of 53 unrelated individuals from France. There is an association between binding affinity to BIP-45 and the XbaI RFLP; the 8.6 kb XbaI allele (absence of cutting site) being associated with low-affinity binding to BIP-45. In this sample of individuals there is no significant association between serum cholesterol levels and BIP-45 binding affinity, but there is a significant correlation between serum cholesterol levels and XbaI genotype, with individuals of the genotype X1X1 having the highest and those with the genotype X2X2 having the lowest levels of serum cholesterol. This suggests that variation at the ApoB locus may be involved independently in the determination of serum lipid levels and in the development of ischaemic heart disease.