Carbohydrates are involved in many essential biological recognition processes in physiological and pathological states. Thus, it is important to understand the mechanism of protein-carbohydrate interactions at molecular level. In the present study, molecular recognition force spectroscopy was applied to investigate the interactions between RCA(120), a lectin from Ricinus communis, and galactose (Gal) and asialofetuin (ASF) at the single-molecule level. RCA(120) coupled to the AFM tip could specifically recognize Gal and ASF, respectively. The unbinding forces of RCA(120)-Gal and RCA(120)-ASF increase linearly with the logarithm of loading rate. The results reveal that the binding capability of RCA(120) toward Gal is weaker than that of ASF, implicating a multivalent effect in the RCA(120)-ASF interaction. (C) 2011 Elsevier Inc. All rights reserved.