A type I ribosome-inactivating protein, extracted and purified from M. charantia seeds, was crystallised by vapour diffusion with polyethylene glycol at pH 7.2. X-ray data were collected to 2.1 A resolution and the structure solved by molecular replacement using the A-chain coordinates of ricin. The overall fold of the protein is similar to ricin but there are differences in secondary structure, on the surface and in the active site cleft. These differences are probably due in part to the evolution of the protein without a B-chain partner. The most extensive reorganisation occurs at the C-terminus whereas Tyr70 shows the greatest change in the active site cleft.