Recently, using an APOE cDNA probe, we discovered an Hpa I restriction fragment length polymorphism (RFLP) that appeared to be strongly associated with the expression of type III hyperlipoproteinemia (Klasen et al. 1987). In the present report it is shown that the same Hpa I RFLP can be revealed with both the APOC1 and APOC2 cDNA probes. This enabled us to localize the polymorphic Hpa I site between the APOE and APOC1 genes and to localize the APOC2 gene approximately 22 kb 3' of the APOC1 pseudogene on chromosome 19.