The paper demonstrates the importance of sequence analysis of DNA for the identification of Hb-Sydney [alpha2-beta2 67 (E11) Val-Ala]. The latter was considered erroneously, based on results of biochemical analyses to be Hb-M-Milwaukee [alpha2 beta2 67 (E 11) Val-Glu]. With the unstable Hb-Sydney correspond also phenotypical manifestations of disease (haemolytic anaemia with Heinz bodies in red blood cells). Sequence analysis of DNA of patients with Hb-Olomouc [alpha 2 beta 2 (F 2) Ala-Asp] revealed that mutation of Ala-Asp in position 86 (F 2) of the beta globin chain is coded by mutation C-->A (GCC-GAC).