Hemoglobin beta Knossos (beta 27Ala-Ser) is a cause of beta-thalassemia due to its reduced synthesis. To investigate the basis for this observation, we have isolated the beta Knossos gene and examined its expression in heterologous cells. We have found that some beta Knossos RNA transcripts are abnormally processed, utilizing a cryptic splice sequence that is enhanced by the Knossos substitution. This form of abnormal RNA processing is seen in two other mutations in this region (a silent substitution in codon 24 and the substitution in codon 26 that produces the beta E variant) and most likely contributes appreciably to the reduced synthesis of beta Knossos.