Nephelium lappaceum L., commonly known as "rambutan," is a typical tropical tree and is well known for its juicy and sweet fruit which has an exotic flavor. Chemical studies on rambutan have led to the identification of various components such as monoterpene lactones and volatile compounds. Here, a 22.5-kDa trypsin inhibitor (N . lappaceum trypsin inhibitor (NLTI)) was isolated from fresh rambutan seeds using liquid chromatographical techniques. NLTI reduced the proteolytic activities of both trypsin and alpha-chymotrypsin. Dithiothreitol reduced the trypsin inhibitory activity of NLTI at a concentration of 1 mM, indicating that an intact disulfide bond is essential to the activity. NLTI inhibited HIV-1 reverse transcriptase with an IC50 of 0.73 mu M. In addition, NLTI manifested a time- and dose-dependent inhibitory effect on growth in many tumor cells. NLTI is one of the few trypsin inhibitors with nitric oxide-inducing activity and may find application in tumor therapy.