The rare apolipoprotein C-II (apoC-II) mutation, apoC-IILys19-->Thr, also known as apoC-II-v, has been found previously in association with hyperlipoproteinemia. From a lipid clinic screening we identified three unrelated individuals who had the apoC-IILys19-->Thr mutation. Among eight family members of one proband, we have found another four who were affected. None of the individuals in this kindred is dyslipidemic and there is no difference in lipid levels between affected and unaffected family members. Therefore, we conclude that the presence of this apolipoprotein variant by itself has no effect on lipoprotein levels. In addition, the apolipoprotein E (apoE) isoform, apoE4 does not have a synergistic effect on lipoprotein levels in this kindred, in contrast to observations on the interaction of apoE4 with another apoC-II mutant (apoC-IIToronto). The single nucleotide substitution-that causes the apoC-IILys19-->Thr variant introduces a previously unrecognized restriction site (for Mae III), that provides for easy screening.