SummaryIn one female and one male patient, an auto-antiM antibody was detected during pretransfusion testing at 4°C. Neither of the patients had developed signs of cold agglutinin disease.The antibodies showed dosage effect, were pH dependent and belonged to the IgM class. Absorption-elution studies with autologous cells, proved the anti-M were real auto-antibodies.The direct antiglobulin test was negative in both patients.One patient was followed for 13 months. During this period, fluctuation of the antibody titer and its thermal amplitude were observed