The purified glycosylphosphatidylinositol phospholipase D (GPI-PLD) is a single-polypeptide with a molecular weight of about 1OO kD. However, the enzyme is eluted in the fraction of 500 kD when human serum undergoes gel filtration chromatography. To study the natural state of GPI-PLD might help to visualize its physiological function. By using gel filtration, hydrophobic column chromatography and ultracentrifugation, and analyzing the concentration of phospholipids, triglycerides and cholesterol, we found that GPI-PLD did not exist as polymer of polypeptids in serum, but combined with the serum lipids to form a complex of lipids and proteins. As a result, it is present in a density zone similar to HDL after ultracentrifugation, this complex exists in serum separately from the subfractions of HDL which are abundant in Apo-A1. In addition it could bind on the heparin-Sepharose affinity chromatography column as Apo-E.