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 non-inhibitory concentrations in the aqueous humour, while therapeutically effective levels of the antibiotic appeared in the aqueous after subconjunctival administrationof tobramycin (mean: 18.9 microgram/ml). These data suggest that tobramycin sulphate may be of value in the treatment of ocular infections.