Data are presented on the relative antimicrobial activities of seven commonly used chemical compounds that are used for preserving ophthalmic drug solutions from Pseudomonas and Proteus contaminations. In testing the compounds against 26 strains of Pseudomonas aeruginosa and four species of Proteus in a simple buffer solution, in the absence of an ophthalmic drug, the order of activity of the agents from high to low were found to follow the pattern: Benzalkonium chloride > phenylmercuric nitrate > phenol or chlorobutanol > parabens > thimerosal > phenylethyl alcohol. When the compounds were tested against some of the organisms in distilled water the order of activity from high to low was found to be as follows: Benzalkonium chloride > phenylmercuric nitrate > chlorobutanol > phenol or thimerosal > parabens > phenylethyl alcohol. When the preservatives were added to aqueous solutions of ophthalmic drugs experimentally contaminated with Pseudomonas aeruginosa, the order of activity of the compounds was: Benzalkonium chloride > chlorobutanol > phenol > parabens > phenylmercuric nitrate > phenylethyl alcohol or thimerosal. Several suggestions are given for the selection of a suitable preserving agent for ophthalmic drugs and certain procedures to be followed in the formulation of solutions under practical conditions in hospital and pharmacy routine.