Increased activity of blood‐group substance A was found over a three‐month period in the serum of a Negro who was group A1and a secretor of A and H. Two of his children, A1B secretors, possessed the same feature, but two other children (A1and A1B secretors) did not. The three persons with increased serum values for A all had hemoglobin S; the two others did not. However, there does not appear to be a close association between the two traits because the sera of A, secretors who were parents of children with sickle‐cell anemia contained normal amounts of A act