Basement membranes have been implicated in morphogenesis and cell differentiation. In this study, the effect of basement membrane components on intestinal epithelial cell maturation in a mesenchyme-free environment was investigated. Fetal rat small intestinal epithelial cells (from the 14th-17th day of gestation) were exposed to basement membrane-derived proteins (laminin, collagen type IV, and a complex basement membrane-enriched extract from the Engelbreth-Holm-Swarm sarcoma) and other extracellular matrix proteins (collagen type I and fibronectin) coated onto Petri dishes. The cells attached readily only to fibronectin and basement membrane proteins. For 5 days the developing epithelial colonies were monitored in vitro, assessing morphological and functional parameters of cell maturation. Colonies grown on laminin and the basement membrane extract were larger and of greater cell density. An increase in alkaline phosphatase and lactase activity was observed after 3-4 days in these colonies which could be enhanced to yield 90%-100% positive cells by the addition of dexamethasone to the medium while no sucrase-isomaltase activity was elicited. Electron microscopy confirmed a high degree of cellular polarization illustrated by tight junctions and apical microvilli in epithelial cells grown on a basement membrane-like support. In contrast, none of the other proteins stimulated the cells to mature in vitro. The authors conclude that certain basement membrane components actively promote fetal intestinal epithelial cell differentiation.