The interaction of rare Hb variants with beta(0)-thalassaemia results in a quasihomozygous state where the erythrocytes contain the variant as the only major adult Hb component. Such a situation is a unique model that enables functional studies even in the case of a neutral variant that could not be isolated from Hb A. We report here an unusual patient carrying Hb Arta, a novel Hb variant [beta 45 (CD4) Phe-->Cys], in trans with beta(0)-thalassaemia gene (beta(0) 39). The aminoacid substitution at the critical CD corner of this Hb molecular renders the molecule unstable. In addition, haem is displaced in a position that favours the deoxy (T) conformation of the variant, but less than in Hb Cheverly [beta 45 (CD4) Phe-->Ser], and results in a p50 of 43 mmHg (pH 7.4, 37 degrees C) in the red cells with preservation of cooperativity. Solution studies of the almost pure Hb Arta show a 50% decrease in oxygen affinity and normal cooperativity; the Bohr effect and the interaction with organic phosphates are similar to those of Hb A. Hb Arta retains both normal homo- and heterotropic effects allowing a well-preserved oxygen transport in vivo despite a mild anaemia.