Using dimethyl sulfate and kethoxal, we have probed antibiotic-ribosome complexes, and identified sites of interaction of chloramphenicol, erythromycin, carbomycin, vernamycin B and viomycin with 23S rRNA. Chloramphenicol, erythromycin, carbomycin and vernamycin B protect overlapping nonequivalent sites in the central loop of domain V. From the known functional effects of these drugs and their protection patterns, we infer that peptidyl transferase is inhibited as a result of binding antibiotics proximal to A-2451, whereas antibiotics bound proximal to A-2058 interfere with growth of the nascent polypeptide chain. Vernamycin B also strongly protects A-752, implying that this region of domain II is proximal to the central loop of domain V. Viomycin, which affects translocation and subunit dissociation, protects U-913 and G-914.