In this work we consider the differential effect of Valinomycin used at different concentrations both on the protein synthesis of reticulocytes and on 42K exchange. We demonstrate that there is a two step action of this antibiotic. At 10-6M and below the drug has no effect on the 42K exchange, but it stops, however reversibly, protein synthesis. At 10-5M the drug has a very sharp action on the 42K exchange and stops protein synthesis in an irreversible way. Ribosomal population checked by two ways, sucrose gradient and direct counting on E.M. sections shows that at low concentrations of Valinomycin (10-8M to 10-6M) there is no breakdown of the polysomes which can be detected by either one of these methods. On the contrary, after short incubation with 10-5M of Valinomycin the breakdown of ribosomes is very clear, as evidenced by sucrose gradient analysis. By direct ribosomes clusters counting on E.M. sections this breakdown is seen only after long incubation.