Fatty acid-binding proteins (FABP) are small cytosolic proteins which are thought to play a key role in fatty acid metabolism. The intestine contains the intestinal (I-FABP) and the liver (L-FABP) isoforms, but their regulation is still poorly documented. In order to find suitable conditions for studying the regulation of the two FABP isoforms in Caco-2 cells, we investigated the effects of the presence of collagen during cell proliferation or differentiation. When collagen was present only during cell proliferation on culture dishes, I-FABP expression was enhanced, whereas sucrase-isomaltase was unaffected and L-FABP expression was merely accelerated. In contrast, when collagen was present during cell differentiation on filter inserts, both I-FABP and sucrase-isomaltase were strongly reduced, but L-FABP was not affected. Under the former conditions (the more suitable for studying FABP regulation), the peroxysome proliferator-activated receptor (PPAR) activators, clofibrate and alpha-bromopalmitate, enhanced the two isoforms. This study, which is the first one providing a quantitative protein analysis of I-FABP and L-FABP in Caco-2 cells, demonstrates different time courses of expression of these proteins during cell differentiation. It also shows that I-FABP is specifically regulated by collagen and that, under conditions optimal for their expression, both isoforms are modulated by metabolic factors.