The shape of the clasps on removable partial dentures often has to be adjusted by bending. Occasionally they fracture during the small plastic deformation that takes place during the adjustment. The tendency to brittle failure of three Co-Cr-Mo alloys for prosthetic use with different carbon and chromium contents was investigated by tensile testing. The total elongation which can be used as a measure of the ductility was observed to vary from 4-17% for the investigated alloys. The reasons for these differences have been sought by studying the structures of the alloys and analysing the fracture mechanisms. The fractures are basically brittle and seem to be initiated during straining by cracking of particles. Microprobe analyses showed that many of the particles were agglomerations of constituents which in all cases were substantially enriched in C, Cr and Mo. Each of these constituents may contain phases too small to be resolved by SEM and by the electron beam in the microprobe. TEM investigations showed that many of the constituents were eutectic with alternating lamellae of Co-rich FCC structure and M23C6. The crack propagates rapidly either by following the interface between the matrix and the particles or by running inside the particles. Cleavage along stacking faults in FCC Co-rich grains in the matrix takes place when the distance between neighbouring particles increases. The ductility of the alloys is clearly improved by decreasing amounts of particles and thus by a reduction of the C content.