The selAB operon codes for the proteins selenocysteine synthase and SELB which catalyse the synthesis and co-translational insertion of selenocysteine into protein. This communication deals with the biochemical characterisation of these proteins and in particular with their specific interaction with the selenocysteine-incorporating tRNA Sec. Selenocysteine synthase catalyses the synthesis of selenocysteyl-tRNA Sec from seryl-tRNA Sec in a pyridoxal phosphate-dependent reaction mechanism. The enzyme specifically recognizes the tRNA Sec molecule; a cooperative interaction between the tRNA binding site and the catalytically active pyridoxal phosphate site is suggested. SELB is an EF-Tu-like protein which specifically complexes selenocysteyl-tRNA Sec. Interaction with the selenol group of the side chain of the aminoacylated residue is a prerequisite for the formation of a stable SELB·tRNA complex. Mechanistically, this provides the biochemical basis for the exclusive selection of selenocysteyl-tRNA Sec in the decoding step of a selenocysteine-specific UGA triplet.