As a result of Sorsby's reports * suggesting that subconjunctival injections of penicillin could prevent experimental production of endophthalmitis by susceptible organisms, it became the practice of several services at the Illinois Eye and Ear Infirmary during 1949 to administer penicillin subconjunctivally as a prophylactic measure at the completion of cataract extractions. During 1952 streptomycin was added to the subconjunctival preparation to increase its bacterial range. This modification was made because of the unfortunate occurrence during one afternoon of three consecutive cases of postoperative endophthalmitis due to Aerobacter aerogenes, which proved to be penicillin-resistant. A total of 1202 cases had prophylactic penicillin injections, and in 3226 cases the injection contained both penicillin and streptomycin. During the immediately preceding period and the early part of the same period some services at the Infirmary did not use any prophylactic injection, and these cases, totaling 1773, form the control series. TECHNIQUE The penicillin-streptomycin solution