Colistimethate sodium (Coly-Mycin) was used in the treatment of 17 patients: 13 had urinary tract infections (two of these had positive blood cultures), three had respiratory tract infections, and one patient had both urinary and respiratory tract infections. In nine of the 17 a foreign body-either a carcinoma, a catheter, or a stone-complicated the infection.The dosage used was 1.1-2.3 mg./lb./day with a maximum in one case of 2.4 g. given over an eight-day period. The organisms so treated included Pseudomonas, six; Aerobacter, six and E. coli, two. Both Pseudomonas and Aerobacter were encountered in three cases.On bacteriological grounds, six patients were cured, eight relapsed, and in three the infecting agent was replaced by another organism. The best responses were obtained in those patients with Pseudomonas infection. Side effects included nausea, vomiting, vertigo, paresthesias, and pain at the site of injection.Colistimethate sodium has a place in the treatment of Gram-negative infections excluding Proteus organisms.