Abstract
Anti-M antibodies are usually of IgM, appear as cold agglutinins and are clinically insignificant. We are reporting two cases of anti-M in cases of solid tumors where the anti-M caused discrepancy in blood grouping, reacted in coombs phase of crossmatching. Anti-M in first case showed dosage effect. These antibodies can be clinical significant when detected in coombs phase, making M antigen negative coombs compatible unit transfusion imperative.