Although the tRNA species directing selenocysteine insertion in prokaryotes differ greatly in their primary structure from that of their eukaryotic homologues they share very similar three-dimensional structures. To analyse whether this conservation of the overall shape of the molecules reflects a conservation of their functional interactions it was tested whether the selenocysteine inserting tRNA species from Homo sapiens supports selenoprotein synthesis in E. coli. It was found that the expression of the human tRNA(Sec) gene in E.coli can complement a lesion in the tRNA(Sec) gene of this organism. Transcripts of the Homo sapiens and Xenopus laevis tRNA(Sec) genes synthesised in vitro were amino-acylated by the E.coli seryl-tRNA ligase although at a very low rate and the resulting seryl-tRNA(Sec) was bound to and converted into selenocysteyl-tRNA(Sec) by the selenocysteine synthase of this organism. Selenocysteyl-tRNA(Sec) from both eukaryotes was able to form a complex with translation factor SELB from E.coli. Although the mechanism of selenocysteine incorporation into seleno-proteins appears to be rather different in E.coli and in vertebrates, we observe here a surprising conservation of functions over an enormous evolutionary distance.