This study shows the results of in vitro globin chain synthesis analysis in 33 infants who had been previously evaluated for the presence of thalassemia in the second trimester of gestation and were restudied after the stage of hematological maturity. Four children with alpha-thalassemia-1, identified in a newborn screening, were also included. Normals and beta-thalassemia heterozygotes could be distinguished in the neonatal period by beta/alpha or beta/gamma ratios. However, as a considerable overlap of alpha-thalassemia-1 with normals and alpha-thalassemia-2 with beta-thalassemia heterozygotes were found, biosynthetic studies at birth seem to be inappropriate to make reliable diagnosis of hemoglobin chain deficiencies. There were no differences between hematological indices of normal and heterozygous beta-thalassemia newborns, while alpha-thalassemia-1 carriers showed a statistically significant difference from normals in mean MCV and MCH.