1. alpha(A)-Chain sub-units have been isolated from neutral haemolysates containing certain unstable beta-chain haemoglobin variants. They are identical, in electrophoretic properties, tryptic peptide analyses and their ability to recombine with Hb-beta(A) (4) at neutral pH, with the alpha(A)-chain sub-units isolated from Hb-A at acid pH. Abnormal alpha(G)-chain sub-units have been prepared from haemolysates containing the alpha-chain variant Hb-G. 2. alpha(A)-Chain sub-units prepared by either method are eluted from Sephadex at low haemoglobin concentration as monomer sub-units. The elution volume, sedimentation coefficient and apparent molecular weight are, however, concentration-dependent, suggesting an equilibrium between monomers and higher polymers with the former as the predominant species. Elevated temperatures cause the alpha(A)-chain sub-unit to aggregate and denature. 3. The product of the reaction between the alpha(A)-chain sub-unit and Hb-beta(A) (4) has been characterized as Hb-A. Only Hb-A and Hb-G can be detected as the products when a mixture of alpha(A)- and alpha(G)-chain sub-units reacts with Hb-beta(A) (4). The absence of the species alpha(A)alpha(G)beta(A) (2) can be explained in terms of the rapid equilibrium between whole and half haemoglobin molecules and the reassortment of alphabeta sub-units during the separation process.

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