The molecular basis of a mutation conferring an erythromycin-resistance phenotype was explored, as an approach to the role of 23 S rRNA in the peptidyl-transferase activity of 50 S ribosomal subunits. Mutagenization of an Escherichia coli strain, which carried the multicopy plasmid pLC7-21 containing the rrnH operon, led to the production of an erythromycin-resistant strain. Plasmid pBFL1 isolated from this mutant was able to transform the sensitive RecA- strain EM4 and to induce a "dissociated" type of antibiotic resistance. Two ribosome populations occurred in EM4/pBFL1: normal particles coded for by the seven rrn chromosomal genes and mutated particles containing rRNA of plasmid origin. The latter particles displayed in vitro lower affinity and susceptibility to erythromycin than wild type particles. The mutation within plasmid pBFL1 was mapped by a multiple primer extension technique. Three synthetic primers were used to sequence the central loop in domain V of 23 S rRNA, leading to identification of a C to U transition at position 2611. This base change was proved to be responsible for the erythromycin-resistance phenotype by the plasmid-plasmid marker rescue technique. A molecular explanation for the rrn mutations leading, respectively, to undissociated and to dissociated types of resistance to the MLSb (macrolide-lincosamide-synergimycin B) group of antibiotics is proposed. These results and some literature data support the notion that rRNA bases involved in antibiotic resistance play a conformational role in the ribosomal binding sites for the MLSb antibiotics.