A report of a doubly heterozygous case for both haemoglobins: Hb Stanleyville II (alpha 78 replaced by Lys) and Hb S (beta 6 Glu replaced by Val) with Hb hybrid S/St II. A 29-year-old woman from Zaïre was found to have four haemoglobins: Hb A, Hb Stanleyville II, Hb S and Hb hybrid S/St II. The clinical and hematological effects of this combination were similar to those of heterozygous sickle cell anaemia. Substitution of Lysin for Asparagin at residue alpha 78 reduced the tendency to polymerization and increased mechanical stability. These findings demonstrated involvement of this site in intermolecular interactions and explained the moderate severity of the sickle cell anaemia syndrome.