Electron microscopy of freeze-dried, shadowed fibrin fibers has demonstrated that these structures are twisted. The pitch and radius of many fibers were measured from the micrographs. Although there is some variability, the average pitch of 1930 +/- 280 (SD) nm is independent of radius. The distribution of observed radii of fibers assembled in vitro is highly skewed, suggesting that individual fibers grow to a maximum radius of about 50 nm, except when both pH and ionic strength are high; fibers aggregate to form thicker fiber bundles under some conditions. The observed twisting may be responsible for limiting the lateral growth of individual fibers. Protofibrils near the surface of a twisted fiber are stretched relative to those near the center. Consequently, the degree to which a protofibril can be stretched limits the radius of a fiber; protofibrils can be added to a growing fiber until the energy required to stretch an added protofibril exceeds the energy of binding. These properties of assembly arise directly from the intrinsic twist of the fibrinogen molecule determined from structural evidence. Simple geometric considerations lead to conclusions regarding the locations of the binding sites for assembly of the protofibril and the flexibility of the fibrin molecule.