Studies by Struble and Bellows1 and Leopold and LaMotte2 on the penetration of penicillin into the eye after topical and systemic use were carried out with commercial preparations, and the concentrations obtained in various parts of the eye were expressed in Oxford units. A direct comparison of their results and those published by this laboratory3 is not possible, since in our experiments a relatively crude preparation of penicillin was applied. The strain of Penicillium notatum used was grown in the medical department of the Presbyterian Hospital under the supervision of Dr. Gladys Hobby and extracted in the laboratory of Dr. Karl Meyer, of the Department of Ophthalmology; titration according to the Oxford method was not done. In the same study, the penicillin activity in the aqueous and vitreous fluids was determined by means of the Fleming dilution method, and the concentrations were given in micrograms of penicillin