The effect caused by medium acidification up to pH 6.5 on the agglutinating ability of 7 anti-A MAbs (2-8, 2-17, 2-19, 2-22, 2-23, 2-28 from Workshop IV and BRIC-145) and their inhibition by glycoconjugates obtained from the membranes of A1 erythrocytes by enzymatic treatment and the chloroform-methanol method, followed by ion-exchange gel chromatography, was evaluated. Medium acidification most significantly reduced the agglutination of A1 erythrocytes in IgM MAbs 2-17 , 2-19, 2-22, and 2-28 and had a weaker manifestation in more alkaline IgM MAbs 2-8, 2-23, and BRIC-145. The inhibition of the lipid isotypes A(1p-00), A(1p-0), and A(1p-3) (with the isoionic pH points of 8.1, 8.0, and 6.55) and the protein ones A(pr-1), and A(pr-3) (with the isoionic pH points of 7.15 and 6.45) was assessed in scores. Acidification up to a pH value of 6.5 in MAbs 2-28 and 2-17 caused a considerable reduction in inhibition with acid A(1p-3) and A(pr-3) with slightly increased inhibition with alkaline A(1p-00); MAbs 2-22 and 2-19 insignificantly and selectively altered the inhibitory capacity by more alkaline types of glycoconjugates; MAb 2-8 hardly changed inhibition. All the above illustrates a significant part played in the study effect by both the charge of glycotopes and antibodies and specificity--the selective avidity of MAbs to certain isotypes of A glycotopes.