m Use of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide to introduce RNA-protein crosslinks in the 40S and 60S subunits of the cytoplasmic ribosome of Tetrahymena thermophila is described, and proteins linked covalently to 17S and 26S ribosomal RNAs are identified. RNA-protein crosslinking is accompanied by extensive dimerization and aggregation of ribosomal subur, its probably due to formation of interparticle protein-protein crosslinks.