The invention relates to a novel ribosome-binding protein derived from Bougainvillea spectabilis having a molecular weight of about 26,000 daltons by polyacrylamide gel electrophoresis under reducing and non-reducing conditions, a pI of about 9.0, and a specified amino-terminal amino acid sequence. The invention further relates to a conjugate of the protein with a targeting ligand, such as an antibody, to form an immunotoxin. The protein and the conjugate are useful in therapy, for example, in the control of tumour cells or viruses.