Hemoglobin J Mexico has been found in five generations of a large Algerian family. Nine subjects have 55% Hb J although their parents, siblings and offspring may have 31%, the usual quantity found in heterozygotes. Those with 55t Hb J are considered to be homozygous for a chromosome carrying both a normal a chain locus and a locus forαJ. The proportion of the abnormal hemoglobin in all the subjects is in favor of an unequal expression of both loci, the amount of protein synthesis dissected by theαJgene being greater than that directed by theαA. In two heterozygotes a slightly higher proportion af the Hb J (38%) suggests the presence of a angle normal a chain locus in trans. An associatedα-thalassemia was excluded by biosynthetic studies in this family.