A mild hemolytic anemia in a 43-yr-old black woman was attributed to the presence of an abnormal hemoglobin (Hb Rush) which migrated cathodically to Hb A at pH 8.0. Its structural abnormality was found to be in the β-chain, β101 (G3) glu → gln. Another electrophoretic band at pH 8.0 proved to be a hybrid tetramer (αA2βAβRush). Hb Rush is heat unstable. A likely explanation of the instability is the presence of an uncovered positive charge in the central cavity where normally glutamic acid in position 101 neutralizes arginine in position 104 contributing to the net neutrality in this region. This neutrality is disturbed by the substitution of glutamic acid by glutamine in Hb Rush.