AbstractOut of 11,480 blood donors, 5,380 (46.9%) belonging to a blood group O were screened for the presence of anti‐A and anti‐B haemolysins. Strong haemolysins (complete lysis of A and/or B cells) were detected in 1,739 (32.3%) of these blood donors. A striking observation was the high frequency of these haemolysins amongst ethnic groups originating from the southern part of Nigeria. Possible causes and consequences during blood transfusion and pregnancy are discus