A new litholytic mixture of mono-octanoin (MO) and methyl tert-butyl ether (MTBE) in a ratio of 2:1 (v/v) was employed in 42 patients with bile duct stones, 29 of them failures after papillotomy. Twenty-two of these patients had complicated stones. The new solvent mixture was given for 4-6 h/day and 2-3 ml were instilled every 30 min. Gentle aspiration and instillation were alternated so as to "stir" the preparation around the stones. Perfusion was given for up to six days. The mixture contributed to success in 37 cases (88%), 19 of them with complicated stones. Total dissolution was attained in 18 cases, and in the other 19 cases clearance was achieved after partial lysis followed by easy crushing with a basket. The mean volume of solvent perfused (+/- SD) was 84.9 +/- 39 ml (range 25-150), the mean duration of treatment was 16.5 +/- 7.4 h (range 5-30). Hospital stay averaged 4.6 +/- 1.6 days (range 2-7). There were five failures: in one patient eight large concretions were eliminated only after extracorporeal shock wave lithotripsy (ESWL). Two were re-operated and pigment stones were found. The last two refused alternative treatments. Side effects were minimal and easily managed by withdrawal of a few ml of bile. Treatment with the new solvent may be indicated as first-instance therapy in place of ESWL, laser endoscopy, electrohydraulic or mechanical lithotripsy for patients with complicated biliary stones, or in cases in which endoscopic papillotomy has failed.