The level of Hb C in the erythrocytes of individuals with Hb C trait decreases significantly in the presence of coexisting alpha-thalassemia. This relationship may result from the higher affinity of beta A than beta C for limiting amounts of alpha-globin during hemoglobin assembly. This mechanism would predict that the beta A and beta C synthetic capacity in alpha-thalassemic individuals with Hb C trait should be balanced despite the low levels of Hb C in their circulating erythrocytes. To directly test this prediction, we have measured the beta A and beta C synthetic capacity of reticulocyte RNA isolated from two individuals with Hb C trait, one with a normal alpha-globin genotype and one with alpha-thalassemia. The balanced expression of beta A and beta C in both cases supports the proposed posttranslational control over Hb C expression.