Drug-dependent antibodies can rarely cause interference in pretransfusion antibody screening. The diluents for commercial reagent red blood cells contain different antibiotics, such as chloramphenicol, neomycin sulfate, and gentamycin as a preservative. The presence of antibodies to a given drug in patient may lead to positive results when performing antibody identification. We present a rare case of detection of anti-co-trimoxazole antibody during routine antibody screening in a female patient undergoing neurosurgery. These antibodies mimicked as antibody against high-frequency red cell antigens reacting in both saline phase as well as antiglobulin phase. Anti-co-trimoxazole antibody was confirmed by repeating antibody screen using reagent red cells of different manufacturers with and without co-trimoxazole drug as preservative as well as using washed red cell panels. There were no associated clinical or laboratory evidence of hemolysis.