Alpha-globin genes were analyzed by the direct method of DNA mapping using alpha- and zeta-globin specific probes in a Thai family in which the proposita was an unusually mild beta zero-thalassemia homozygote. alpha zero-Thalassemia was found to be segregating in the family, inherited from the proposita's father by one of her younger sisters. However, alpha zero-thalassemia was not detected by this DNA mapping in the proposita. The mild homozygous beta zero-thalassemia in this family may result from interactions of a non-deletion alpha-thalassemia, a gene responsible for high proteolytic activity permitting more balanced globin-chain levels, or from an unusually active hemoglobin F production in the proposita.