More than 80% of rapidly labelled RNA is found to be bound with cytoplasmic membranes of E. coli and only 17% was discovered in cytoplasmic fraction. 96% of DNA was found in the cytoplasmic membrane fraction. The treatment of E. coli homogenate with sodium deoxycholate resulted in the release of rapidly labelled RNA from membranes. The analysis of membrane-bound rapidly labelled RNA in sucrose density gradient (10-40%) revealed that it sedimented in the polyribosome region. Incubation of E. coli cells with actinomycin D for 10 min at 37 degrees C resulted in the decrease of membrane-bound radioactivity in the polysome region (31% as compared with 55% in the control culture which was incubated for 20 sec with 14C-orotate). The inhibition of protein synthesis with chloramphenicol or puromycin interrupted the decomposition of polysomes in the presence of actinomycin D. The radioactivity of polysomes in these experiments was 50% and 47% respectively and was comparable with the control. Under dialysis of rapidly labelled RNA fraction against buffer with minimal concentration of Mg2+ (0,1 mM) the elimination of the label from the polysome region was observed. The prolongation of the incubation of cells with actinomycin D resulted in the decrease of protein synthetising ability of the membrane fraction. The increase of the inhibition level of protein synthesis by chloramphenicol was shown to result in the increase of the amount of membrane-bound rapidly labelled RNA, which remained stable in the presence of actinomycin D. A number of arguments of the hypothesis on the role of protein in the initiation mechanism of the decomposition of mRNA in bacteria is discussed.