The 3' untranslated region (3' UTR) of α-striated tropomyosin(αTM) promotes differentiation of nonmuscle mesenchymal cells into a muscle phenotype. The extent of this response is cell-type specific, with fibroblasts being able to differentiate further along the muscle pathway than osteoblasts when transfected with the 3' UTR of αTM. Transfected fibroblasts elongate, become multinucleate, and express the muscle-specific proteins titin, tropomyosin, and myosin heavy chain in a sarcomeric distribution. These characteristics are highly reminiscent of striated muscle, and are not observed in untransfected and vector-transfected fibroblasts. To investigate whether the mechanism of this differentiation event involves the production of myogenic transcription factors, RNA was harvested from avian embryonic fibroblasts that were transfected with the 3' UTR of αTM, contained within a eukaryotic expression vector. Control RNA was harvested from vector-transfected and untransfected fibroblasts. RNA was reverse-transcribed and the resultant cDNA was used as template for PCR amplification. RT-PCR analysis reveals the presence of the muscle-specific transcription factors MyoD and myogenin in untransfected, vector-transfected, and 3'UTR transfected groups. This suggests that the 3'UTR of αTM promotes differentiation of fibroblasts into muscle via a mechanism that is not dependent on de novo production of the muscle transcription factors MyoD and myogenin, unless it acts by increasing the amount of these factors present within transfected cells. We speculate that cell types containing low levels of muscle transcription factors (e.g. fibroblasts) are poised to enter the muscle differentiation pathway when transfected with the 3' UTR of αTM.