Fine epitope specificity of ten monoclonal antibodies (MA) agglutinating red blood cells B was studied. Three methods were used: 1) inhibition of MA binding to natural antigen by synthetic oligosaccharides (OS) and their polyacrylamide conjugates, 2) direct MA binding to a series of synthetic OS-polyacrylamide conjugates differing in carbohydrate epitope density, 3) direct MA binding to the affinity sorbents. It is shown that all antibodies studied prefer trisaccharide B determinant Gal alpha 1-3(Fuc alpha 1-2) Gal independently of their ability to discriminate serological subgroups of B erythrocytes (B, Bweak, B3). The correlation of the MAs epitope specificity with their ability to agglutinate red blood cells B subgroups is discussed. Of an interest is that MAs which are able to agglutinate any B subgroups also bing the synthetic tetrasaccharide Gal alpha 1-3(Fuc alpha 1-2)Gal beta 1-3GalNAc, a B type 3 determinant.