Using p-nitrophenylcarbamyl-phenylalanyl-tRNA (PNPC-Phe-tRNA) and N-Iodoacetylphenylalanyl-tRNA as affinity labels we have attempted to identify the components of the aminoacyl-tRNA binding sites located in the vicinity of the peptidyl transferase centre of the yeast ribosome. Both Phe-tRNA derivatives bind to the ribosomal A-site in the presence of 20 mM Mg++ ion concentration and can be translocated to the ribosomal P-site in the presence of elongation factor. After the labels have been allowed to react covalently with ribosomes they were found associated with the large ribosomal subunit. Proteins L36, L43, L42, L29, L2, L17/18, L19/20 and proteins L26, L38, L22/23, L7/9, L4/6, L36, L11, L43, L39 were labelled in samples treated with PNPC-Phe-tRNA and N-iodoacetyl-Phe-tRNA respectively. In contrast, when only the components of the ribosomal P-site were analysed by reacting the treated particles with puromycin fewer spots were labelled, corresponding to proteins L36 and L19/20 using PNPC-Phe-tRNA and proteins L4/6, L36, and L43 using N-Iodoacetyl-Phe-tRNA.