THE majority of human red cell cold agglutinins have a specificity for an antigen termed I which is present in the red cells of the vast majority of adults¹. Only very occasionally does an adult lack the I antigen. The high titre anti-I cold agglutinins found in association with chronic autoimmune haemolytic anaemia and, transiently, in association with Mycoplasma pneumoniae infection are therefore autoantibodies. In human infection with Mycoplasma pneumoniae the cold agglutinin seems to be directed not against the infecting organism but against the host red cell². The present study was undertaken to investigate whether I antigen can be detected in human tissues other than erythrocytes and hence to determine whether anti-I cold agglutinin is an organ specific or a non-organ specific antibody.