The alpha 2-globin gene from a patient with alpha thalassemia contains a pentanucleotide deletion in intron 1 immediately adjacent to exon 1. We tested the functional consequences of this mutation by introducing the thalassemic gene, along with its normal counterpart as a control, into cultured monkey cells on SV40 plasmid vectors. Both genes are expressed, at similar levels, into globin RNA with the correct 5' and 3' ends. However, while most of the normal transcripts are appropriately processed, the thalassemic transcripts are abnormally spliced from a 5' donor site in the middle of exon 1 to the normal 3' acceptor site. This results in the synthesis of a truncated RNA incapable of encoding a normal globin polypeptide. The alternative donor, also used at a low level in monkey cells transfected with the normal gene, shows strong homology to the consensus donor sequence characteristic of many eucaryotic splice junctions. No unspliced or partially spliced thalassemic RNA was detected, indicating that recognition of this site is efficient and does not block removal of intron 2. The alternatively spliced RNA was also found in bone marrow RNA from the alpha-thalassemic patient, although not in that from a normal individual or a beta +-thalassemic patient. The thalassemic phenotype of the patient therefore results from abnormal RNA splicing owing to the deletion of the first splice donor signal.