1. Thymosin alpha 1 precursor [3H]cDNA-cellulose synthesis was carried out by reverse transcription with RNA-dependent DNA polymerase from avian myeloblastosis virus using oligo(dT)-bound to cellulose as a primer. 2. Unlabelled thymosin alpha 1 precursor cDNA-cellulose was synthetized in a preparative scale to be used in affinity chromatography. 3. Poly(A)-mRNA from calf thymus was subjected to cDNA-cellulose affinity chromatography and an active messenger obtained in a yield of 3% respect to the total thymus poly(A)-mRNA chromatographied. 3. The analysis of the translation products of the purified mRNA have shown it as the thymosin alpha 1 precursor messenger.