The intestinal epithelium provides an excellent starting material for the isolation of natural microfilament organizations in amounts suitable for biochemical studies. The microvillus filament bundle core and the terminal web provide two distinct microfilament systems. We review the current knowledge of the filament bundle core and the attempts that have been made to reconstitute this structure from actin and its four major associated proteins. We show in addition that a high molecular mass actin-binding protein (TW-260/240) having spectrin-like properties is, next to myosin, the major associated protein of the terminal web retained in isolated brush borders. We summarize the biochemical and morphological evidence for the existence of a class of spectrin-related molecules in the cortical cytoplasm of many cell types. These findings may lead to a new understanding of membrane-microfilament interactions.