Beta-globin gene mutations which alter normal globin RNA splicing have confirmed the necessity of invariant nucleotides GT at donor splice sites. Functional consequences of point mutations in the invariant AG acceptor splice site have not been determined. We have isolated and characterized a beta-globin gene from a Black patient with beta-thalassemia intermedia which has an A-G transition at the usual intervening sequence 2 (IVS2) acceptor splice site. Functional analysis of transcripts produced by this mutant gene in a transient expression vector indicates that the mutation inactivates the normal acceptor splice site and results in some utilization of a cryptic splice site near position 580 of IVS2. This mutation would be expected to produce a beta-globin gene which results in no normal beta-globin mRNA.