W e tested whether apoprotein B is present in fasting and postprandial human duodenojejunal mucosa because lipoprotein-like particles are visualized by electron microscopy within the smooth endoplasmic reticulum and the Golgi cisternae of these absorptive cells. Duodenojejunal biopsies from normal volunteers were incubated in citrate buffer and were shaken in 1% EDTA so that absorptive cells could be freed from underlying tissue. Apoprotein B was determined by double-antibody radioimmunoassay in homogenates of absorptive cells. The preparations of absorptive cells were shown to be uncontaminated by plasma lipoproteins; they did not contain any albumin by immunodiffusion able to detect 2 mug/ml. They adsorbed less than 0.1% of 125I-low density lipoprotein which was added to the citrate buffer. Cell preparations from suction biopsies of human rectum contained no detectable apoprotein B. Duodenojejunal absorptive cells from 22 fasting subjects contained 3.2 +/- 0.5 mug of apoprotein B per 100 mg (wet wt) of biopsies or 1.3 mug of apoprotein B per mg of total cell protein. The amount of apoprotein B per milligram of cell protein fell to 0.3 mug in 14 of these individuals whose mucosa was also sampled 45 min after instilling fat intraduodenally. These experiments provide immunochemical evidence that human duodenojejunal absorptive cells contain apoprotein B. This technique should be valuable for studying the physiology of intestinal lipoproteins in absorption and in patients with hyperlipidemia.