The metabolic fate of 1 of 3H/14C-labeled dilinoleoglycerophosphocholine was studied in five patients after oral administration. The 3H label was in choline and 14C was in the two linoleic acid residues. More than 90% of both isotopes was absorbed from the intestine. Seventy to 90% of the 3H radioactivity in blood was linked to phosphatidylcholine (PC) whereas 14C was associated with both PC and nonpolar lipids. At peak activity, the 3H/14C ratio of plasma PC was twice that of oral PC; this suggests that most oral PC was hydrolyzed to lysolecithin before absorption. The mean maximum concentration in total blood volume was 20% of the administered dose for 3H and 28% for 14C. Examination of lipoproteins revealed that the specific activity of PC in high density lipoprotein (HDL) was 2 to 6 times higher than in apoB-containing lipoproteins, and to 2 to 20 times than that of red blood cells or total blood. Thus, absorbed PC seemingly was incorporated preferentially into the HDL fraction of plasma.