FIELD: medicine.SUBSTANCE: present invention relates to medicine, specifically to laboratory diagnostics, and can be used to detect a malignant tumour expressing CAPRIN-1. Method comprises measuring the expression of a polypeptide capable of binding, via an antigen-antibody reaction, to an anti-CAPRIN-1 protein antibody having any of the amino acid sequences with even numbers SEQ ID NO:2–30 from the sequence listing, wherein the expression of the polypeptide on the surface of the tumour cell is measured by immunoassay of an antibody which may be contained in the sample and induced in vivo against the measured polypeptide.EFFECT: invention provides tumour detection in a patient expressing CAPRIN-1.9 cl, 4 dwg, 5 ex