THE human haemoglobin (Hb) variant Constant Spring (Hb CS) occurs in a number of racial groups and is involved in the aetiology of as many as 50% of all cases of Hb H disease, a disorder which presents a major public health problem in south-east Asia1,2. It has an elongated α chain of 172 residues which has probably arisen by mutation of the normal α-chain-terminating codon (UAA) at position 142 to a glutamine codon (CAA), allowing read-through into extrastructural codons at the 3′-end of the α mRNA3. We report here the characterisation of Hb Icaria, a second Hb with an elongated α chain of 172 residues. Its structure is identical with that of Hb CS except that residue α 142 is lysine instead of glutamine.