Tropocollagen was extracted from the extracellular fibers of the cell layers of cultured human fibroblasts and procollagen was isolated from the medium. The tropocollagen precipitated as segment long spacing (SLS) aggregates from an ATP-acetic acid solution, but procollagen formed predominantly fibrous long spacing (FLS) crystallites in the same solution. The procollagen aggregates were distinguished from those of tropocollagen by the presence of globular caps at one (SLS form) or both (FLS form) ends. The globular caps were selectively removed by limited digestion with pepsin. They are thought to be formed from the nonhelical, aminoterminal peptides of the procollagen molecule. These globular peptides coil back 200 A on the long axis of the molecule and contribute no more than 90 A to its length. They appear to prevent the molecules from aggregating as native fibers in vitro.