Clinical manifestations and hematologic data of thalassemia intermedia were observed in three siblings of a Thai family. Analyses of the hemoglobin of their parents and other siblings indicated that they inherited a delta beta-thalassemia gene from the father and a beta zero thalassemia gene from the mother. Globin gene mapping confirmed that they carry two abnormal beta-globin gene complexes. On one chromosome more than 70 kb of DNA was removed which resulted in G gamma (A gamma delta beta)zero-thalassemia. The deletion started at the Hind III site located just 3' to the G gamma gene, and extended downstream to a region recognized by the p3'N 2.8R probe which is located more than 45 kb from the 3' end of the beta gene. The other chromosome carried a beta zero thalassemia gene, and a 5 kb deletion between the G gamma and A gamma genes which produced a hybrid -GA gamma- gene. A synthetic oligonucleotide probe showed that this beta zero thalassemia arose from a C----T mutation at position 654 of IVS-II in the beta-globin gene.