The localization of the Р185^HER2 transmembrane receptor in SKOV-3 and BT-474 cancer cells was studied by fluorescence, confocal, and electron immunomicroscopy. The P185^HER2 receptor is a marker of breast and ovarian tumors; it is also considered to be a target for anticancer therapy. It is extremely important to choose a universal immunicytotoxic agent applicable, firstly, to study the distribution of Р185^HER2 in cancer cells, secondarily, to remove P185HER2 from the cell surface and, thirdly, to eliminate target cells. In this study, for visualization Р185^HER2 we propose an immunocytotoxic system, which consists of monoclonal miniantibody 4D5 scFv to the extracellular Р185^HER2 domain fused with two molecules of barnase (cytotoxic RNAase from Bacillus amyloliquefaciens) and its specific inhibitor, barstar. Fluorescent microscopy showed that the module 4D5 scFc-dibarnase:barstar is efficient for identifying Р185^HER2 on the surfaces of cancer cells. It was found by confocal microscopy that interaction with 4D5 scFc-dibarnase results in the internalization of Р185^HER2. The localization of Р185^HER2 in human ovarian carcinoma cells (SKOV-3) and breast carcinoma cells (BT-474) was compared by electron microscopy using 4D5 scFv-dibarnase:barstar-Au and 4D5 scFv-dibarnase-Au complexes. Р185^HER2 is distributed unequally on the cell surface with preferential localization on protrusions or close to their bases and at contacts between protrusions and the cell membrane. At 37°C, Р185^HER2 is internalized through coated pits and vesicles and concentrates in endosomes and multivesicular bodies in the cells of both cell lines, as well as in lysosomes in BT-474 cells.