A case of chronic cold haemagglutinin disease due to an IgM monoclonal (kappa) autoantibody with anti-M-like specificity is described in a patient with the MN phenotype. The autoantibody was present in very high titre and active at body temperature, but haemolysis was only mild. The direct antiglobulin test was positive due to C(3)d on the patient’s red cells, and the autoantibody was able to bind complement to normal MM and MN cells with a marked dosage effect.