Hb Bab-Saadoun which has a Leu→Pro substitution at position 48 of theβchain was detected in a young Arabian boy living in Tunisia. His parents did not have the variant which suggests that it occurred as a spontaneous mutation. The substitution is located in the interhelical CD segment; leucine atβ48 is an invariable amino acid that may be important as part of a spacer sequence between the two helices and its replacement by proline may affect the stability of the hemoglobin molecule. Hb Bab-Saadoun is unstable in heat and isopropanol stability tests and its chain was best isolated by parachloromercuribenzoate precipitation. It appears unlikely that the presence of Hb Bab-Saadoun results in a hemolytic anemia.