Sodium dodecyl sulfate-acrylamide gel electrophoresis and molecular-sieve chromatography on 8% agarose demonstrate the existence of a very high molecular weight (500,000-600,000), proline-rich protein in cultured 3T6 fibroblasts that appears to be the precursor molecule (procollagen) of collagen. The kinetics of [(3)H]-proline uptake indicate that this precursor is synthesized at a different rate than are other cell proteins and is secreted apparently unchanged into the medium, where it undergoes modification; it then precipitates around the cells as collagen fibrils that contain the characteristic tropocollagen polypeptide chains. The solubility of this precursor in hot 5% Cl(3)CCOOH, its hydroxyproline to proline ratio, and its sensitivity to highly-purified bacterial collagenase all indicate that this molecule is of collagenous nature, but that it has considerable regions of noncollagen peptide (about half of the molecule is collagenase sensitive, and it has half of the normal amount of hydroxylated proline residues). These results support the concept of a procollagen molecule, of molecular weight about 500,000-600,000, that contains large regions of noncollagen peptides, which might allow the collagen alpha-chain regions to associate in register and that also can provide the correct chain composition for tropocollagen. Upon secretion of the procollagen molecule, the intercollagen-peptide regions are cleaved and the finished tropocollagen molecule then polymerizes into typical intercellular fibrils.