Hydrogel coated ring shaped ocular inserts (containing the antibiotic, tylosin tartrate) were used in an evaluation of the effectiveness of polymeric ocular drug release devices for treating infectious bovine keratoconjunctivitis. Two major in vivo tests were done: a treatment test using infected cattle and a test to evaluate ring retention characteristics. The in vivo experiments represent the first experiments using hydrogel ocular inserts containing an antibiotic for treating infectious bovine keratoconjunctivitis;The bacterium, Moraxella bovis, was used to replicate the disease. In the infection tests, ten calves were challenged with 2.4 x 10[superscript]8 - 1.6 x 10[superscript]9 M. bovis colonies per eye following two ten minute ultraviolet radiation eye preconditioning exposures. Ninety five percent of the eyes (19 of 20 eyes) were successfully infected by this method. The first measurements were performed at day 3 of the tests. For 14 days, all infected eyes were monitored for the presence of the bacteria quantitatively. Clinical observations were also made. At day 7 after inoculation of both eyes of a calf with M. bovis, a medicated or a non-medicated ring-shaped device was inserted into each eye of a calf. The eye receiving the non-medicated ring was taken as a control for comparison with the eye that received a medicated ring. During the next 7 day period following a medicated ring insertion, the number of bacteria in the treated eye dropped dramatically to negligible levels (0 to 30 colony forming units/swab), while the control eye which received a non-medicated ring still exhibited a relatively high number of bacteria (10[superscript]3 to 10[superscript]6 colony forming units/swab). The number of bacteria was significantly reduced by the antibiotic released from the medicated ocular insert;In the retention test, the optimal size criterion of a ring was determined using a two-tailed t-test in order to select the proper size range for rings so that they would be retained for a week. By using the criterion, 1.18L < ring size < 1.23L, where L is the side-to-side dimension of the palpebral aperture of the eye, 56 of 70 eyes (for 38 cows) retained (non-medicated, hydrogel coated) rings for at least 7 days. Also, the success rate for the population (any large group of cattle) was estimated as approximately 70% to 90% (P < 0.05).