The present invention is related to the obtaining of modified antibodies by means of DNA recombinant technology from the murine monoclonal antibody P3 (MAb P3), produced by the hybridoma cell line deposited under Budapest Treaty with accession number ECACC 94113026, and from its anti-idiotype murine monoclonal antibody 1E10(MAbai 1E10), produced by the hybridoma cell line with deposit number ECACC 97112901, with the objective of achieving monoclonal antibodies which preserve the biological function of specifically binding the antigen of the original antibodies, but being at the same time less immunogenic. The chimeric antibodies of the invention contain the variable domains of the murine immunoglobulin and the constant regions of the human immunoglobulin; and those humanized, besides containing the constant regions of the human immunoglobulins, are modified in the murine framework regions (FRs), in particular, in those zones that could be in a T cell antigenic site, so that several positions of the FRS are human as well. These antibodies can be used in the diagnosis and therapy of different types of tumors. The present invention is also related to the use of the antibodies for therapeutic and diagnostic purposes.