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The association-dissociation of ribosomes is caused by changes in the concentration of magnesium ions; this transformation does not involve profound conformational changes as revealed by absence of hyperchromic effect [l] , of any alterations of the ORD spectra [2,3] or of a difference in susceptibility to attack of SH-reagents [4] . Nevertheless, a remarkable difference of the conformations of RNA in associated and dissociated subunits of E. coli ribosomes has been found recently with the use of circular dichroism in the UV-region [I]. However, the latter studies did not attempt to evaluate the contribution of conformational changes with the separated subunits. The increase of dichroic absorption upon formation of 70 S particles was regarded as a direct consequence of the interaction of the contact areas of the subunits. The present communication is concerned with the circular dichroism (CD) spectra of ribosomes fixed with glutaraldehyde [5]. It was found that changes in the concentration of magnesium ions result in a change of CD spectra of flYted ribosomes similar to that observed with intact ribosomes although the former are incapable of association-dissociation. Hence, change of the CD spectrum reflects some changes of the conformation of rRNA caused by change of environment and is not a direct consequence of the association-dissociation.