A family was studied who carried a slow mobility haemoglobin on cellulose acetate electrophoresis at pH 8.6. The structural analysis of the anomalous globin chain showed substitution of residual aspartic acid in position 94 of the alpha chain by tyrosine (Hb Setif). This mutation induces low oxygen affinity in the haemoglobin molecule plus instability of the tetramer in the oxy conformation. Such haemoglobin has been found in North-African populations, and the case presented here is the first one reported in Spain.