A 22 year old American negro with mil.d anemia was found to be triply heterozygous for henoglobin S. hemoglobin 0 Arab and alpha thalasseaia. Hemoglobin A was not detected in the subject's hemolysate. The alpha thalasseaia gene was expressed by anα/non-αsynthesis ratio of 0.71à0.07 and was equally unbalanced in the peripheral blood and the bone marrow. The total radioactivity of the abnormal chains was equal, indicating equal rates of transcription and translation of the abnormal structural genes. Since the rate of synthesis of BSand B0was equal to the proportions of these abnormal chains in the peripheral blood, the abnormal gene products were equally stable. The mildness of the anemia, compared to that of the other reported S-O Arab double heterozygotes, may be due to the moderating influence of the alpha thal gene.