Immune electron microscopy with a panel of monoclonal antibodies to O. felineus antigens and human immune sera from patients was used for localization of the main antigens of adult O. felineus. The immune complexes at the ultrastructural level were visualized by 5-nm colloidal gold. The main antigens recognized by human antibodies and monoclonal antibodies were associated with the tegument, muscles, uterus, gonads, intestine and eggs of the liver fluke. The findings led to the conclusion that the surface structures of liver flukes stimulate a low B-cell immune response. The structures of the excretory-secretory system of the parasite and their products contain a lot of main antigens and induce B-immune response in man.