We determined the relative potencies of penicillin G, carbenicillin, oxacillin, cephalothin, cephaloridine, gentamicin, kanamycin, neomycin, vancomycin, and bacitracin after their addition to three commercially available 0.5% hydroxypropyl methylcellulose artificial tear solutions in plastic squeeze bottles. We found no significant loss of antibiotic activity at room temperature over a period of seven days for all antibiotics except penicillin G, cephalothin, and cephaloridine. Three of the antibiotics were insoluble in one or more of the artificial tear solutions.