Sialic acid-binding lectin (SBL) isolated from Rana catesbeiana eggs is a basic protein which agglutinates a large variety of tumour cells and has an amino acid sequence homologous to that of human angiogenin and pancreatic ribonuclease (RNase). Although SBL and angiogenin lack the Cys-65-Cys-72 disulphide bond of pancreatic RNase, the locations of the other three disulphide bonds are similar among the three molecules. SBL was found to exhibit RNase activity, as well as catalytic properties resembling those of bovine RNase A in some respects. For example, SBL hydrolyses poly(uridylic acid) and poly(cytidylic acid) as substrates, and prefers the former. RNase A and angiogenin are strongly inhibited by human placental RNase inhibitor, whereas the RNase activity and tumour cell agglutination activity of SBL are not affected by this inhibitor.