A method for the preparation of heavy-atom derivatives is described. It has succeeded with yeast cytochrome c peroxidase and may be useful for other crystalline proteins. Transfer of the crystals from methylpentanediol solutions (in which they were grown) to solutions of polyethylene glycol (MW 6000) maintains the crystal structure so that crystals tolerate heavy-atom concentrations at least 10 times larger than could be attained in methylpentanediol mother liquors.