Apoproteins have important physiologic functions in lipoprotein metabolism. Several apoproteins are produced in the intestine including ApoA-l, ApoA-IV, and ApoB. Each appears to participate in intestinal lipid transport. The liver also produces several apoproteins, including ApoC-II and ApoC-III, but the data demonstrating the ability of the intestine to produce ApoC is incomplete. Our aim was to ascertain whether ApoC-II and ApoC-III were present in human and rat jejunum, and if so, whether their presence was altered by fat feeding. The technique of immunolocalization and a newly developed double antibody radioimmunoassay for rat ApoC-IIIa were used. ApoC-IIIa was found in the supranuclear regions of enterocytes along the entire lengths of villi in the jejuna of 12-h-fasted rats. 1 hour after the gastric ingestion of corn oil, ApoC-llla was found primarily in between cells and in the lamina propria. Similar results were obtained in human jejunal biopsies with ApoC-ll and ApoC-lll. ApoC-llla was also detected by radioimmunoassay in enterocytes isolated from jejuna of neonatal and adult rats. Thus, ApoC-ll and ApoC-lll are clearly present in the intestine as well as in the liver. In addition, because their localization is altered after fat feeding, they are also likely to be produced in the enterocyte.