Erythrocyte superoxide dismutase (SOD; E.C.1.15.1.1) levels were quantitated in human haemolysates, both by an immunological technique and by an activity assay. The erythrocyte SOD activity level in a group of seven Fanconi's anaemia patients was significantly decreased (27%, on the average) compared to a control group of nineteen healthy individuals. The fact that both the activity per antigenic unit and the electrophoretic mobility of the erythrocyte SOD was normal indicates that the deficiency in these Fanconi's anaemia patients is most probably not due to a mutation in the structural gene for the enzyme but more likely due to a disturbed mechanism regulating the SOD level in the erythrocyte.