Photoreactive derivatives of E. coli tRNAPhe bearing arylazido groups on guanine residues (azido-tRNA) were used for affinity labelling of E. coli ribosomes in the region of the P-site when the A-site was either free or occupied by aminoacyl-or peptidyl-tRNA.Corresponding complexes of azido-tRNA with ribosomes and poly(U) were obtained both nonenzymatically and with the use of elongation factors. UVirradiation of the complexes resulted in labelling of ribosomal proteins (preferentially of 30 S subunit). Proteins S9 and S21 were labelled only when the A-site was free; S14 -only when it was occupied; Sl 1,