Described herein are antibodies, and antigen-binding fragments thereof, that are specific for tumor endothelial marker 1 (TEM-1), related polynucleotides, expression vectors, and cells that express the described antibodies. Also provided are methods of using the described antibodies, and antigen-binding fragments thereof, and related kits. Provided herein are also methods for diagnosing TEM-1-expressing cancers (such as melanoma, sarcoma, bladder cancer or colon cancer) using the described antibodies, and antigen-binding fragments thereof. The methods involve determining the amount of TEM-1 in a biological sample derived from a subject and comparing this level with the level of TEM-1 in a known standard or reference sample.