Chylomicrons produced by the human gut contain apolipoprotein (apo) B48, while very low-density lipoproteins made by the liver contain apo B100. To study the function of these molecules, we examined lipid absorption occuring in apobec-1 knockout mice (able to produce only apo B100; KO) and in wild type mice (of which the normally functioning intestine makes apo B48, WT). Using the lymph fistula model, we studied the process of lipid absorption when animals were intraduodenally infused with a lipid emulsion (4 µmoles/h or 6 µmoles/h of triolein). KO mice transported triacylglycerol (TG) as efficiently as WT mice when infused with the lower lipid dose; when infused with 6 µmoles/h of triolein, however, KO mice transported significantly less TG to lymph than WT mice, leading to the accumulation of mucosal TG. Interestingly, lipoprotein particle size from both KO and WT mice were enlarged to chylomicron size particles during absorption of the higher dose. These increased-size particles produced by KO mice were not associated with increased apo AIV secretion. However, we found that the gut of KO mice secreted fewer apo B molecules to lymph, both during fasting and lipid infusion, leading us to conclude that the KO gut produced less number of TG rich lipoproteins than the wild type animals. Reduced apo B secretion in KO mice was not related to reduced MTP lipid transfer activity. We propose that apo B48 is the preferred protein for the gut to coat chylomicrons to ensure efficient chylomicron formation and lipid absorption.