An isolated monoclonal antibody that binds to PD-1, which contains a heavy chain variable region containing CDR-H1, CDR-H2 and CDR-H3 amino acid sequences, and a light chain variable region containing CDR-L1, CDR-L2 and CDR-L3 amino acid sequences, wherein: (i) the heavy chain variable region contains a CDR-H1 sequence selected from SEQ ID NO: 1, a CDR-H2 sequence selected from SEQ ID NO: 3, and a CDR-H3 sequence selected from SEQ ID NO: 5; and (ii) the light chain variable region comprises a CDR-L1 comprising the amino acid sequence of SEQ ID NO: 7, a CDR-L2 comprising the amino acid sequence of SEQ ID NO: 9, and a CDR-L3 comprising the amino acid sequence of SEQ ID NO: 11; wherein the antibody is a murine or chimeric antibody wherein the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 13, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 14; or wherein the antibody is a humanized antibody, wherein: (a) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 17, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 18; (b) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 19, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 20. a heavy chain variable region containing CDR-H1, CDR-H2 and CDR-H3 amino acid sequences, and a light chain variable region containing CDR-L1, CDR-L2 and CDR-L3 amino acid sequences, wherein: (i) the heavy chain variable region contains a CDR-H1 sequence selected from SEQ ID NO: 1, a CDR-H2 sequence selected from SEQ ID NO: 3, and a CDR-H3 sequence selected from SEQ ID NO: 5; and (ii) the light chain variable region comprises a CDR-L1 comprising the amino acid sequence of SEQ ID NO: 7, a CDR-L2 comprising the amino acid sequence of SEQ ID NO: 9, and a CDR-L3 comprising the amino acid sequence of SEQ ID NO: 11; (i) the heavy chain variable region contains a CDR-H1 sequence selected from SEQ ID NO: 1, a CDR-H2 sequence selected from SEQ ID NO: 3, and a CDR-H3 sequence selected from SEQ ID NO: 5; and (ii) the light chain variable region comprises a CDR-L1 comprising the amino acid sequence of SEQ ID NO: 7, a CDR-L2 comprising the amino acid sequence of SEQ ID NO: 9, and a CDR-L3 comprising the amino acid sequence of SEQ ID NO: 11; wherein the antibody is a murine or chimeric antibody wherein the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 13, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 14; or wherein the antibody is a humanized antibody, wherein: (a) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 17, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 18; (b) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 19, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 20. (a) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 17, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 18; (b) the heavy chain variable region comprises the amino acid sequence of SEQ ID NO: 19, and the light chain variable region comprises the amino acid sequence of SEQ ID NO: 20.