There is a single-site interaction of [methylene-14C]thiamphenicol and [methylene-14C]chloramphenicol with run-off ribosomes with dissociation constants Kd = 6.8 micronM and Kd = 4.6 micronM respectively. Similar affinities for the antibiotics are observed in polysomes totally deprived of nascent peptides, or bearing nascent peptides on the A-site. However, two types of interaction are observed in endogenous polysomes with some ribosomes bearing nascent peptides on the P-site and other in the A-site. The lower-affinity bindings (dissociation constants Kd = 6.4 micronM and Kd = 1.5 micronM for thiamphenicol and chloramphenicol respectively) are due to the ribosomes bearing nascent peptides on the A-site. The higher-affinity bindings (dissociation constants Kd = 2.3 micronM and Kd = 1.5 micronM for thiamphenicol and chloramphenicol, respectively) are due to the ribosomes bearing nascent peptides on the P-site. Therefore binding of nascent peptides to the A-site does not affect the affinities of thiamphenicol and chloramphenicol for the ribosome. On the other hand interaction of the nascent peptides with the P-site of the ribosomes increases the affinities of both antibiotics for the ribosome. Thiamphenicol and chloramphenicol are thus good inhibitors of peptide bond formation in ribosomes and polysomes. Their affinities are increased precisely when the peptidyl-tRNA is placed in the P-site preceeding the peptide bond formation step, which is specifically blocked by the antibiotics. There is a single-site interaction per ribosome for [35S]thiostrepton, which does not appear to be affected by the attachment to the ribosomes of mRNA, tRNA and nascent peptides either to the A or the P-site. [N-methyl-14C]Lincomycin, [N-methyl-14C]erythromycin, [G-3H]streptogramin B and [G-3H]-streptogramin A bind to run-off ribosomes and polysomes totally free from nascent peptides. However, these antibiotics do not interact with ribosomes bearing nascent peptides either in the A or the P-site and therefore are not active on preformed polysomes. Thus lincomycin and streptogramin A only interact with free ribosomes and 50-S subunits and block the early rounds of peptide bond formation prior to polysome formation. Erythromycin and streptogramin B do not inhibit either initiation or the first round of peptide bond formation. However, erythromycin and streptogramin B, prebound to the ribosome, block peptide elongation probably by steric hindrance with the growing oligopeptide chain when this reaches a certain critical length.