The extent of the inhibitory effect of ricin in polyphenylalanine synthesis by eukaryotic ribosomes is strongly dependent upon the concentration of ribosomes and the elongation factors EF 1 and EF2. Maximal inhibition by ricin, in this assay is observed when either ribosomes or the two elongation factors are added under limiting conditions, whereas ricin-treated ribosomes support protein synthesis at saturating concentrations of elongation factors and ribosomes. Similarly, the enzymatic binding of Phe-tRNA to ribosomes is drastically blocked in ricin-treated ribosomes when low EF 1 concentrations are added to the reaction mixture, but there is no inhibition when EF 1 is at saturating concentrations. Furthermore, formation of the complex EF 2-guanosine triphosphate-ribosome, using free ribosomes pretreated with ricin, is strongly inhibited at limiting concentrations of EF2, but is not affected at saturating concentrations of this factor. However, ricin does not inhibit the EF 2-dependent translocation of peptidyl-tRNA by polysomes, although the toxin is very active in preventing amino acid incorporation by polysomes. Our results suggest that the damaging effect of ricin on the ribosome causes a decreased affinity for both elongation factors EF 1 and EF 2. Thus, the toxin inhibits the enzymatic binding of aminoacyl-tRNA to ribosomes. The lack of inhibition of translocation by ricin suggests that the toxin cannot interact with ribosomes with substrate bound to the acceptor site. Essentially similar results are observed with ricin, abrin, ricin A chain, abrin A chain, and ricinus agglutinin A chain. A possible effect of the toxins on initiation and/or termination is further discussed.