The ID50 values for azithromycin and clarithromycin inhibition of translation and of 50S assembly in Staphylococcus aureus cells have been measured. For clarithromycin, 50% inhibition of growth occurred at 0.075 mu g/ml, and the effects on translation and 50S formation were equivalent at 0.15 mu g/ml. The inhibition of these processes by azithromycin was less effective, with an ID50 of 2.5 mu g/ml for growth and 5 mu g/ml for inhibition of translation and 50S formation, The additive effects of each of these drugs on translation and 50S formation account quantitatively for their observed influence on cellular growth rates, In macrolide-treated cells, there was also a direct relationship between the loss of ribosomal RNA from the 50S subunit and its accumulation as oligoribonucleotides. These results are compared with the previously described effects of erythromycin on these same processes.