Methylated ribosomal proteins from Escherichia coli 50S subunit are localized by growing cells in a medium containing (1-14C)methionine and (3H-methyl)-methionine and comparing the 3H/14C ratio for each of the 50S ribosomal proteins. The following proteins are methylated: L11, L1, L3, L5, L7, L8, L9, L12, L18, and L33. The nature and stoichiometry of the methylated amino acid(s) in each of the methylated proteins are determined. Protein L11 is the most heavily methylated of all the 50S subunit proteins. This protein has previously been implicated in the peptidyl transferase reaction during protein synthesis (K. H. Nierhaus and V. Montejo (1973), Proc. Nat. Acad. Sci. U. S. 47, 1588-1602). Three proteins (L1, L3, and L5) have intermediate levels of methylation and contain about 0.4-0.6 methyl groups each per molecule of protein. Five other proteins (L7, L8, L9, L12, and L18) are also methylated to a slight extent (-0.1 methyl group/molecule of protein). One unknown methylated neutral amino acid was detected in protein L11 and at least one and possibly two other unidentified methylated amino acids appeared to be present in protein L33.