A quartz crystal microbalance (QCM) biosensor system for lectin-carbohydrate interactions has been developed. Yeast mannan was immobilised on polystyrene-coated quartz crystals, and interactions tested with the lectin concanavalin A (Con A). The biosensor could be easily operated, where mannan immobilisation and all binding analyses were performed in real-time using a flow-through system. The apparent binding constant for yeast mannan to Con A was estimated to be 0.4 microM, well in accordance to reported literature values. In addition, the effective concentration values (EC50-values) for a series of mannose/mannoside ligands, acting as competitors to the mannan/Con A interaction, were determined to range from 0.18 to 5.3 mM, in good correlation with a related enzyme-labelled lectin assay (ELLA) protocol.