Antibodies and antigen-binding fragments of antibodies that bind GCC are disclosed. The antibodies bind an extracellular domain of GCC and can be internalized. In some embodiments, the antibodies are humanized, chimeric or human. Nucleic acids and vectors encoding the antibodies or portions thereof, recombinant cells that contain the nucleic acids, and compositions comprising the antibodies or antigen-binding fragments are also disclosed. The invention also provides therapeutic and diagnostic methods utilizing the antibodies and antigen-binding fragments provided herein.