In vitro and in vivo studies are presented to test the hypothesis that the synergistic action of the pristinamycins is not due to a catalytic effect of pristinamycin IIA (PIIA) on the bacterial ribosome. We demonstrate that there is a proportionality between the quantity of PIIA bound on the ribosome and pristinamycin IA (PIA) retained by it. Moreover in vitro and in vivo experiments correlated to biological effects (growth and protein synthesis) demonstrate that pristinamycin IIA is tightly bound on 70S ribosome, which satisfactory explains the so called "lasting damage effect".