Amikacin sulfate was given intramuscularly (IM) (7.5 mg/kg) to study its ocular penetration in man. Seventy-three patients with cataracts received a single dose and 35 received two doses given 5 1/2 to 12 hours apart. After a single dose the aqueous humor levels of the antibiotic between two and ten hours ranged from 0.15 to 3.10 mg/L (average and median, 1.0 mg/L). Two doses given 5 1/2 to eight hours apart produced an average level of 3.5 mg/L (range, 0.91 to 8.31 mg/L). When the second dose was given nine to 12 hours after the first, the aqueous humor levels were similar to those found for a single dose. Aqueous humor concentrations of 1.0 mg/L of amikacin would be expected to be bactericidal for most gram-negative bacterial pathogens, whereas levels of 3.5 mg/L would inhibit most strains of Staphylococcus aureus and many strains of Pseudomonas aeruginosa.