Fig. (12) Poly(U)-dependent poly(Phe) synthesis activity of human 80S ribosomes in the presence of increasing amounts of four macrolides. The poly(U)-dependent poly (Phe) synthesis activity was measured as described under Methods. The 100% corresponds to the amount of Phe incorporated in the absence of antibiotic. The half-inhibitory concentration (IC₅₀) that is the concentration of antibiotic corresponding to 50% inhibition of the activity of the human 80S ribosomes is 0.37, 2.3, 0.2 and 0.12 μg/ml for erythromycin, azithromycin, clarithromycin and telithromycin, respectively.