A new variant of apolipoprotein E (apo E), named apo E-Kochi, was identified in the sera of a 29-year-old male with hyperlipoproteinemia as characterized by a broad-beta band. The characteristic double bands of apo E were seen in the isoelectric focusing gel of very low density lipoprotein from the proband and three members of his family. Of the double bands from the probands, the more cationic component was identical to ordinary apo E3 and the other anionic band was located at approximately a distance of one-half charge to the anode side. This anionic band is a new electrophoretical isoform of apo E (apo E-Kochi), and the molecular weight by sodium dodecyl sulfate electrophoresis and its antigenicity against anti apo E serum are the same as apo E3. Sequence analysis of lysyl endopeptidase fragments showed that apo E-Kochi differs from normal apo E3 at residue 145, where an arginine residue is substituted for histidine.