Dimethyl suberimidate has been used to study the proximity relations between E. coli 50 S ribosomal proteins. The composition of two cross-linked protein complexes has been determined; one is a quaternary complex containg proteins L2, L20, L27 and L32-33, and the other is a ternary complex containing proteins L2, L13 and L20. The vicinity of these proteins in the ribosome is discussed.