The case record of a patient suffering from infectious mononucleosis, complicated by severe haemolytic anaemia, with overt intravascular haemolysis, is reported. Investigation of such cases usually shows a high titre of cold agglutinins with anti-i specificity. In the case reported, the results of serological investigations were atypical, and suggested that cold agglutinins with anti-L activity (rather than anti-i) were predominant in the aetiology of the haemolysis.