Abstract: The present invention concerns specific anti-anti-idiotypic antibodies which react with anti-idiotypic antibodies which represent an internal image of the antigen CA125 and which also bind to the antigen itself i.e. are specific for the tumour-associated antigen CA125. In particular the present invention concerns a CA125-specific anti-anti-idiotypic antibody which reacts with the anti-idiotypic antibody ACA125. In addition the present invention concerns pharmaceutical compositions containing these anti-anti-idiotypic antibodies for treating tumours that express CA125 and in particular ovarian carcinomas.