IntroductionCold agglutinins are incomplete antibodies present in various clinical conditions. They bind with erythrocytes in low temperature and cause selective aggregation thereby enhancing the possibility of haemolysis. Preoperative detection of cold antibody can help in planning of open heart surgery as in our patient where the mitral valve was replaced under normothermia. Case ReportA 40 -year-old female patient presented to us with class III symptoms of dyspnoea on exertion and palpitation for 2 years. Evaluation revealed tight mitral stenosis with moderate mitral regurgitation. She was in atrial fibrillation. After complete evaluation she was planned for elective mitral valve replacement. Her baseline haematological investigations were within normal limits and her blood group was O positive. The blood sent for cross matching was found to be incompatible with all samples tested. Spontaneous clumping was also seen against normal saline suggesting the possibility of the presence of cold agglutinin. Based on this, studies for spontaneous clumping at various temperatures were carried out. Clumps were seen between 4 0 C and 25 0 C but not at 37 0 C. After serial and complete washing of the sample with normal saline, clumping was not noted even at low temperature. On assay, the cold agglutinin titre was found to be 1:128. Further haematological tests in the form of assays for haemolysis were carried out which were negative. Coombs indirect antigen was weakly positive. Bone marrow examination was normal. Antibody assays to rule out systemic lupus