Rabbits were immunised againstEscherichia coli ribosomes and the partially purified immunoglobulin G fraction had maximum ability to precipitate the ribosomes as well as the extracted ribosomal proteins. By digestion of immuno-globulin G with papain, monovalent Fab fragments were produced. The 70 S ribosome and its subunits (50 S and 30 S) were separately treated with Fab and then tested in the kinetic assay of degradation of ribosomes by ribonuclease I at various Mg2+ concentrations. Treated ribosomes and their subunits were degraded at faster rates than the nontreated ones; the rates in both the control and the treated cases were dependent on the concentration of Mg2+. These results indicate the unfolding of the structure of the ribosome on treatment with antibody fragments, which may be due to the weakening of the interaction between rRNAs and ribosomal proteins.