Monoclonal antibodies that specifically bind ataxia telangiectasia-mutated and RAD3-related kinase phosphorylated at position 1989 (pT1989 ATR), antigen binding fragments, conjugates thereof, and the use of these antibodies, antigen binding fragments and conjugates are disclosed herein. Also disclosed are nucleic acids encoding these antibodies, vectors including these antibodies, and isolated host cells transformed with these nucleic acids and vectors. Methods are also disclosed for using these antibodies, such as to detect pT1989 ATR, or to determine the dose of an agent of use to treat a subject. The antibodies are also of use for identifying ATR inhibitors.