Structural features of the tRNA P h e molecule upon ternary complex formation with the bacterial elongation factor Tu were investigates. Phosphodiester bonds at positions 18 and 34 were found to be labilized in bound tRNA. Conversely, a higher stability of the phosphodiester links at positions 20, 21 and 36 was detected. Using ethylnitrosourea as a chemical probe a conformational change occurring at phosphate position 53 was observed in complexed tRNA. These results are interpreted by a structural rearrangement of the nucleic acid induced by complex formation.