The 50 S subunits or the CsCl-treated 50 S subunits of E. coli Q13 were digested with RNase T1 (EC 2.7.7.26) in various conditions. The digests were fractionated by the method of the “successive” polyacrylamide gel electrophoresis. From these experiments we tried to clarify the relative neighboring positions of the 50 S proteins on polynucleotide fragments and estimate a possible topographical arrangement of 50 S proteins on the 23 S ribosomal RNA.