Résumé : It was the aim of this study to estimate the molecular weight of thyroglobulin mRNA. Polyacrylamide gel electrophoresis in the presence of formamide was used because it has been shown to minimize the influence of secondary structure on molecular weight determinations. An apparent mol. wt of 2.8 x 106 has been obtained. Also a new procedure of 33 S mRNA purification is presented resulting in cleaner preparations.