UV-visible, infrared and Raman spectroscopies are used to study the interactions between two histological dyes and poly-L-lysine and insulin. For both dyes, bonding seems to involve their SO3-groups. However, the NH2 groups of insulin do not seem to be involved as they are in the case of poly-L-lysine. Moreover, the interactions seem specifically intermolecular in nature and independant of the existence of a beta-pleated conformation for the peptide.