Primer-directed mutagenesis was employed to introduce an A2058----G transition in plasmid-encoded Escherichia coli 23S RNA at a site that has been implicated, indirectly, in erythromycin binding. The mutation raises the growth tolerance of cells from 30 to 300 micrograms/ml of erythromycin, and cells grown in the presence of erythromycin contain ribosomes with high levels of mutated 23S RNA. In these cells, wild type 50S subunits 'fall off' the message and are selectively degraded, possibly as a result of an erythromycin-induced conformational change. A fast in vitro poly(U) assay revealed minimal effects of erythromycin on elongation beyond tetrapeptides. We correlated these results with the literature data and concluded that erythromycin acts immediately post-initiation and directly, or indirectly, destabilizes mRNA-bound 70S ribosomes, and prevents their recycling by causing 50S subunit degradation.