Modified neomycin prepared by the ionic coupling on xanthan with an activity of 380 UI/mg was characterized in regard to its in vitro and in vivo release rate and therapeutic action with artificial tear eluent. The dynamic system in vitro release showed that after 4 h, there appears a "zero-order" kinetic. Ophthalmic inserts were prepared from modified neomycin and they are used in treating bacterial conjunctivitis. Sterilization of the conjunctival sac is obtained 12 h after insert administration.