The purpose of this study (Contract No. G0122066) was to investigate the effects of fracture filling and associated alteration on the mechanical behavior of rocks. Results are presented for specimens from the climax and Henderson Mines in Colorado and from the San Manuel Mine in Arizona. The description of the rocks includes color, texture, mineral composition, composition and fabric of fracture fillings, density, and porosity. Unconfined and triaxial compression tests were run on the climax and San Manuel specimens. Sonic velocities were measured during the compression tests. Tentative conclusions were that some relation appears to exist between compressive strength and the following features for all specimens: dynamic young's modulus, sonic velocity, density, rock composition, and veinlets.