The effects of 29 type 1 and 2 type 2 ribosome-inactivating proteins (RIPs) from plants on polyuridylic acid-directed polyphenylalanine synthesis carried out by purified ribosomes from Streptomyces lividans were studied. Only dianthin 32, saporins R1 and R3, momordin I, trichokirin, Hura crepitans RIP 5 from latex, crotins 2 and 3, and PAPs C, R, and S, inhibited polyphenylalanine synthesis. Both the type 2 RIPs ricin and volkensin were ineffective on translation. The magnesium concentration affected the inhibition of translation to a considerable extent. Upon treatment with inhibitory RIPs, extraction of rRNA and further treatment with acid aniline, S. lividans ribosomes released an RNA fragment of about 130 nucleotides. The 5' terminal sequence of this rRNA fragment was 5'-GAGGACCGGGACGGACGAACCUCUGGUGUGCCAGUUGU-3', similar to the sequence obtained in Escherichia coli. This indicates that the most probable molecular action of these RIPs on S. lividans and E. coli ribosomes is the same: depurination of the rRNA at a site relevant to the translation mechanism and that has been highly conserved throughout evolution.