We report the preliminary results of laser lithotripsy of cystine stones accomplished in 2 patients with the use of a new method of photosensitization. Transurethral rigid ureterorenoscopy was performed with an 8.5F instrument to reach 0.8 and 1.8 cm. stones located in the right and left lumbar ureters, respectively. A 320 mu. fiber was then inserted and pushed into contact with the stone. Then, 400 preliminary impulses were released from a pulsed dye laser (504 nm) with energy output at 140 mJ. and at a rate of 5 Hz. without any visible modification of the stone structure. The saline solution used as irrigation fluid was then substituted with a 2% solution of rifamycin. Totals of 95 and 152 impulses released at the same energy output previously used were then sufficient to produce a satisfactory fragmentation of the stone in both cases. Sonography and excretory urography showed no alterations of the treated urinary tract in either case. The use of rifamycin in the irrigation fluid is suggested for laser treatment of hard or nonabsorbent stones, since this substance favors optical coupling and lowers the threshold of plasma formation on the surface of the stone.