Synopsis Substantivity of various (125)I-labelled collagen hydrolysates to virgin or bleached and dyed European hair or human callus was tested under conditions which simulated actual application of such hydrolysates in shampoos or foam baths. Factors which most strongly influence protein substantivity were identified. The molecular properties of the hydrolysate are the most important factor. The hydrolysate which adsorbed best had the lowest overall isoelectric point, the highest pH in solution and the largest content of high molecular weight peptides. The extent of adsorption was also a function of concentration of protein, of the duration of application and of the nature of the components of the formulation. Thus, the combination of shampoo and hydrolysate can be adjusted so that a desired substantivity is attained.