Haemoglobin Constant Spring (Hb CS) is a variant with an elongated alpha-chain associated with an alpha + thalassaemia phenotype. The amount of alpha mRNA relative to beta mRNA in reticulocytes was reduced in carriers of Hb CS by an amount equivalent to the reduction observed in carriers of alpha + thalassaemia. In a patient with Hb CS-H disease there was greater alpha/beta mRNA ratio in bone marrow nuclear RNA than in the peripheral blood. Furthermore, all the alpha mRNA in the patient's peripheral blood was derived from the alpha 1 (alpha A) gene. The data suggest that alpha CS mRNA is unstable and degraded in the cytoplasm. This instability may be due to destabilization of a specific sequence in the 3' non-coding region during translation.