The effects of ricin and alpha-sarcin separately or in combination on the conformation of rat liver ribosomes were investigated by measuring the relative accessibility of individual ribosomal proteins to N-ethylmaleimide after 80S ribosomes were treated with these toxins. By using a double-labelling technique in which ribosomes were incubated with the toxins and then treated with 3H-labelled or 14C-labelled N-ethylmaleimide, it was found that labelling of protein L14 was specifically reduced by treatment with ricin, and that of proteins L3 and L4 by treatment with alpha-sarcin, suggesting that the toxins alter the conformation of ribosomes in the vicinity of these proteins. When ribosomes were treated with both ricin and alpha-sarcin, the extent of labelling of protein L3 was reduced compared to that observed after treatment with alpha-sarcin alone. These results are discussed in relation to previous observations showing that these three proteins are neighbours in the 60S ribosomal subunit and probably play important roles in protein biosynthesis, and in the actions of ricin and alpha-sarcin on 28S rRNA.