A n arginine-rich apoprotein obtained from human triglyceride-rich lipoprotein was isolated on a heparin affinity column when either the aqueousor urea-soluble apoproteins were applied to the column. Of all the aqueous- or urea-soluble apoproteins, only this arginine-rich protein exhibited a binding affinity to heparin. This protein was eluted from the column at sodium chloride concentrations above 0.35 M in the absence of urea and between 0.17-0.2 M when isolated in urea. The protein has been characterized by amino acid analysis, immunoelectrophoresis, dodecyl sulfate polyacrylamide electrophoresis, isoelectric focusing, and NH₂-terminal analysis. It has the same amino acid composition, NH₂-terminal, and molecular weight as previously described for human arginine-rich apoprotein.The triglyceride-rich lipoproteins of fasting normal humans were eluted as two fractions when applied to the heparin affinity column. A small amount was eluted in the unbound fraction and this species contained virtually no arginine-rich apoprotein. The bulk of the triglyceride-rich lipoproteins eluted in the bound fraction and contained appreciable amounts of arginine-rich apoprotein. The bound lipoproteins had more cholesterol and cholesterol ester and less triglyceride than the unbound. The isolated arginine-rich apoprotein was derivatized with phenylglyoxal with a resulting alteration of 75% of the arginine residues. This modified apoprotein did not bind to the heparin affinity column. Similar treatment of the whole triglyceride-rich lipoprotein produced a lipoprotein that was totally eluted in the unbound fraction.