Galectins are a family of endogenous lectins with adhesion growth-regulatory activity binding galactosides. They are also known to play important roles in cancer, inflammation and a lot of other biological processes. Because of such features characterisation of galectin-carbohydrate interactions and identification of binding sites is of crucial importance for drug design. In the present study rat galectin-5 (rGal5) peptides containing key amino acids involved in carbohydrate binding were identified by CREDEX-MS of rGal5 and lactose - in complete agreement with the binding sites of the molecular modelling structure. Until now there are no X-Ray and NMR structures available for galectin-5 therefore studying galectin-5-carbohydrate interaction is very interesting. Both identified carbohydrate-binding peptides were synthesized and their affinity for lactose was demonstrated by affinity-MS.To get a more detailed view on the affinity and interaction kinetics of the CRD peptides and full galectin-5 with lactose SAW-bioaffinity measurements were performed. For real-time study of carbohydrate-lectin interaction a newly developed online-coupling of surface-acoustic wave measurements with ESI-Ion Trap MS was used. With this system the peptides interacting with on chip bound carbohydrate can be directly measured, enabling real time studies of peptide/protein-carbohydrate interactions.