The rate of protein synthesis in HeLa cells was measured at various periods of time after addition of abrin and ricin to the medium and compared with the concurrent ability of the isolated ribosomes to support poly(U)-stimulated synthesis of polyphenylalanine in a cell-free system. Similarly, the endogenous synthesis in unfractionated cell-free systems from HeLa cells and rabbit reticulocytes was compared with the ability of the isolated ribosomes to support poly(U)-stimulated polymerization of phenylalanine. In the intact cells and the unfractionated cell-free systems protein synthesis decreased progressively with the time after addition of toxins or toxin A chains. In contrast, the ability of the isolated ribosomes to support polyphenylalanine synthesis was only moderately reduced initially and then remained constant or even increased. The activity of isolated monosomes decreased progressively with time after addition of toxin A chain, whereas polysomes were only partly inactivated and the extent of inactivation varied from one experiment to another. The results indicate that the inactivation of one or a few ribosomes per polysome stops the translation of mRNA. It is suggested that the intact ribosomes thus trapped are inaccessible to the toxins and that the isolation of polysomes results in release of functionally intact ribosomes capable of supporting poly(U)-directed polymerization of phenylalanine.