We have shown previously that a beta-globin RNA-deficient beta zero-thalassemia is caused by a single base-pair deletion in codon 44 of the human beta-globin gene1. The lack of beta-globin RNA in erythroid cells of these affected individuals is due to extreme beta-globin RNA instability (t 1/2 = 30 min)2. We have further investigated the mechanism of this extreme lability by transiently expressing the beta zero-thalassemic allele in HeLa cells and assaying the stability of the beta-globin RNA that is produced. Surprisingly, the beta zero-thalassemic RNA is much more stable in HeLa cells than in bone marrow cells. Apparently, developing erythroid cells have a mechanism for turning over this thalassemic RNA while cervical carcinoma cells do not. We also have assayed the stability of RNA derived from in vitro-mutagenized beta-globin genes. In HeLa cells, beta-globin RNAs harboring deletions in and around the translation initiation codon accumulate to steady-state levels that are similar to the level of normal beta-globin RNA.