Erythropoietin (Epo) and thyroid hormone (T 3 ) are key molecules in the development of red blood cells. We have shown previously that the tyrosine kinase Lyn is involved in differentiation signals emanating from an activated erythropoietin receptor. Here we demonstrate that Lyn interacts with thyroid hormone receptor-interacting protein 1 (Trip-1), a transcriptional regulator associated with the T 3 receptor, providing a link between the Epo and T 3 signaling pathways. Trip-1 co-localized with Lyn and the T 3 receptor ␣ in the cytoplasm/plasma membrane of erythroid cells but translocated to discrete nuclear foci shortly after Epo-induced differentiation. Our data reveal that T 3 stimulated the proliferation of immature erythroid cells, and inhibited maturation promoted by erythropoietin. Removal of T 3 reduced cell division and enhanced terminal differentiation. This was accompanied by large increases in the cell cycle inhibitor p27Kip1 and by increasing expression of erythroid transcription factors GATA-1, EKLF, and NF-E2. Strikingly, a truncated Trip-1 inhibited both erythropoietin-induced maturation and T 3 -initiated cell division. This mutant Trip-1 acted in a dominant negative fashion by eliminating endogenous Lyn, elevating p27 Kip1 , and blocking T 3 response elements. These data demonstrate that Trip-1 can simultaneously modulate responses involving both cytokine and nuclear receptors.