The strong sequence homology described recently between Gc (Vitamin D-binding protein) and albumin, and the ability of the latter, to bind 2-p-toluidinylnaphthylene sulfonate (TNS) promoted similar binding studies with Gc. TNS bound to native Gc as demonstrated by fluorescence, but chloroform:methanol extraction of Gc and gas chromatography revealed that large amounts of unsaturated fatty acids - 16:1, 18:1, 18:2 and 20:4 were also associated with Gc. In addition, TNS fluorescence was abolished by delipidation of Gc, and restored upon subsequent reconstitution with fatty acid. Finally, 70-80% of [3H]-arachidonic acid added to whole serum bound to Gc. These results demonstrated that TNS fluorescence of the native molecule reflects associated lipid, and suggest a novel role for Gc as a reservoir for a circulating pool of unsaturated fatty acids.