Streptozotocin, an antibiotic derived from Streptomyces achromogenes, was selected for treatment of a previously diabetic patient with incapacitating hypoglycemia caused by an islet cell carcinoma that had metastasized to the liver. Complete control of all symptoms, together with normalization of biochemical measurements, disappearance of evidence of hepatic metastases as shown on liver scan, and return to his pretumor diabetic state, have been accomplished without the development of a serious toxic reaction. This antibiotic appears to be useful in the control of insulinomas not amenable to surgery. Significant laboratory observations over an 80-day period of treatment are presented.