To elucidate the role of fixed charge and chain rigidity on the assembly of amphiphilic compounds over a polymer substrate, we studied the interaction between sodium dodecyl sulfate (SDS) and crystalline or soluble collagen. Shrinkage temperatures and the equilibrium degree of swelling were measured for crosslinked tendons under isoelectric (pH = 6.0) conditions or in acid (pH = 2.5) solutions. Viscosities and solubilities were measured for soluble collagen to construct pseudo phase diagrams delimiting the field of stability of the helical, random coiled, and crystalline forms. Under isoelectric conditions, increasing concentrations of SDS (up to similar or equal to 0.1 M) cause a depression of transformation temperatures which is much larger than that observed with salts, with aliphatic alcohols, or with sodium methyl sulfate. This is attributed to the prevailing hydrophobic interaction of the surfactant with the random coiled form of collagen, as evidenced by adsorption of the surfactant onto the higly swollen, denatured tendon. The cooperativity of the binding process at pH = 6.0 confirms that peculiar micelle-like assemblies are formed by SDS within the disordered random coiled network. Under nonisoelectric conditions the transition temperature is not much affected by conformation. SDS brings about a dramatic collapse of the lateral dimension of tendons at pH = 2.5, as well as precipitation of soluble collagen, even at very low concentrations, revealing a cooperative phenomenon leading to an increased interhelix interaction. The extent of adsorption of the surfactant over the protein, considerably larger at this pH, emphasizes the strength of electrostatic interaction enhanced by the hydrophobic tail of the surfactant. However, poor cooperativity is shown by the binding data at pH 2.5, suggesting that ordered assembly of cylindrical micelle-like aggregates is prevented by the low charge density of collagen. Tendons at low pH might be described as highly swollen oriented liquid crystalline networks and may undergo a phase transition to the crystalline state as a result of the strong binding between fixed charges and dodecyl sulfate ions.