Complementary DNA enriched in sequences hybridizing to beta-globin mRNA was prepared with viral RNA-dependent DNA polymerase and used as a probe for the presence of beta-globin mRNA in nuclear and cytoplasmic RNA from two Italian patients with beta0-thalassaemia. In both cases the beta-globin gene was present and cytoplasmic mRNAbeta was absent; however, one case appeared to transcribe mRNAbeta and to fail to process it, while the other appeared transcriptionally defective. Evidence is also presented that the low levels of hybridization usually found at high RNA/cDNAbeta ratios in beta0-thalassaemia are due to delta-globin mRNA; the melting profile of the hybrid formed has been determined and a low melting temperature relative to mRNAbeta - cDNAbeta demonstrated.