SUMMARY Tobramycin sulphate, a new aminoglycoside antibiotic, was injected intramuscularly (80 and 100 mg) and subconjunctivally (10 mg) into patients, and the concentrations of the drug in serum and aqueous humour were determined 1 hour after the injection by an agar wall diffusion plate method. The intramuscular administration of tobramycin produced insignificant and noninhibitory concentrations in the aqueous humour, while therapeutically effective levels of the antibiotic appeared in the aqueous after subconjunctival administration of tobramycin (mean: 18 9 ,g/ml). These data suggest that tobramycin sulphate may be of value in the treatment of ocular infections.Aminoglycosides are broad spectrum antibiotics widely used in the treatment of ocular infections and for postoperative prophylaxis. Tobramycin sulphate is a new water-soluble, parenterally administered antibiotic of the aminoglycoside group, derived from Streptomyces tenebarius. Its chemical structure, kinetics, pharmacodynamics, and antimicrobial properties closely resemble those of gentamicin (Bodey and Stewart, 1972;Dienstag and New, 1972). It has been proved that tobramycin is more effective than gentamicin against pseudomonas strains in vitro (Traub and Raymont, 1972; Levison and Kaye, 1974;Wretlind and Gezelius, 1976), and it has been reported that it seems to be less toxic than gentamicin (Federspil, 1974). The peak serum concentration of tobramycin occurs between 30 and 90 minutes after intramuscular administration, and detectable levels persist for as long as 6 to 8 hours (Goodman and Gilman, 1975 The patients were divided into 3 groups as follows: In the first group each one of 9 patients was given 80 mg of tobramycin in a single intramuscular injection; in the second group each of 7 patients was given 100 mg in a single intramuscular dose; in the third group each of 15 patients was given 10 mg of tobramycin subconjunctivally in the eye to be operated upon. The eye was anaesthetised with benoxinate 0 4% eye drops, and 0.25 ml (10 mg) of the tobramycin solution was injected subconjunctivally in the inferior half of the globe by means of a tuberculin syringe with a 27 G needle. The patients in all 3 groups received the antibiotic 1 hour before operation. The eyes were prepared for routine cataract extraction. On the operating table, after retrobulbar anaesthesia, lid block, and conjuctival flap dissection, a keratome making a 3 mm incision was inserted in the anterior chamber at 12 o'clock. Simultaneously a tuberculin syringe with a 25 G needle was introduced into the chamber below the keratome and 0-2 ml of aqueous humour was aspirated. At the same time a venous sample (4 to 5 ml) was also collected. The operation was continued in routine fashion. The blood and 660 copyright.