A 13 kDa protein, covalently linked to a small RNA from the cytoplasm of mouse cells, was studied. Sequence analysis of its tryptic peptides revealed that the RNA-linked protein is identical to prothymosin a. Very similar RNA-protein complexes were identified in human, bovine and yeast cells. Tryptic peptide maps of 12SI-labelled RNA-linked proteins of diverse origin demonstrated their marked similarity, thus indicating high evolutionary conservation of prothymosin a from yeast to man.Protein-RNA complex; Prothymosin a; Evolutionary conserved protein