The intraocular penetration of a new antibiotic, ceftriaxon, was studied in 30 patients. Serum samples in 29 patients showed an average level of 42 micrograms/ml. Penetration into the aqueous humour flow studied in 28 patients under general anesthesia at one, two, three, four and six hours after the injection. An average level of 0,53 micrograms/ml was found which is 1,25% of average serum level. In 23 patients the dry level in tears averaged 5,75 micrograms/ml (13% of the serum level). In one case of retinal detachment the level in the subretinal fluid was 1,56 micrograms/ml. The kinetics drug secretion in tears was studied in 3 patients without anesthesia every 2 hours until the 16 th to 22 nd hours. Lower levels than those obtained under general anesthesia were obtained. This study shows that ceftriaxon has an intraocular penetration at levels above the minimal inhibitory concentrations for most commun intraocular infection.