Following intravenous infusion of an 8 g single dose of the antibiotic agent fosfomycin, its penetration into human primary aqueous humor was investigated. 1-2 h after the start of infusion, peak values of 35 to 60 micrograms/ml were reached. These concentrations in the aqueous humor would inhibit a high percentage of gram-positive and gram-negative strains commonly causing bacterial endophthalmitis. The infusion of an 8 g dose of fosfomycin immediately preceding intraocular surgery thus constitutes effective peri-operative prophylaxis. Moreover, as a broad-spectrum antibiotic, fosfomycin is indicated as initial therapy in intraocular bacterial infections.