Polymeric collagen fibrils have been reacted with fluorescein and rhodamine isothiocyanates to produce fluorescent dye-labelled fibrils, containing seven dye substituents per molecule of tropocollagen within the polymeric collagen fibrils. Two dye-labelled peptides per molecule of tropocollagen were solubilised by trypsin (EC 3.4.21.4) from the telopeptide regions and four dye-labelled peptides were located in the helical regions solubilised by bacterial collagenase (EC 3.4.24.3). The solubilisation of dye-labelled peptides from these insoluble substrates were employed to measure the kinetics of trypsin and collagenase digestion of the telopeptide and helical regions, respectively, of the insoluble polymeric collagen fibrils. These studies demonstrated an apparent excess of enzyme for the readily available substrate under conditions when it was known that a vast excess of substrate existed in the reaction mixture calculated in terms of a molecular ratio. A point of equivalence was established for both trypsin and bacterial collagenase, approximately one enzyme molecule per 870 substrate molecules. On either side of this point the quantity of products formed was controlled by either the enzyme concentration or the substrate concentration. The results can be explained in terms of the inaccessibility of tropocollagen molecules within the molecular architecture of the polymeric collagen fibrils. The external layer of tropocollagen molecules obstruct collagenolytic enzymes penetrating to, and forming enzyme-substrate complexes with, the bulk of the substrate within the interior of the fibrils.