The occurrence of methylated proteins in the ribosomes of Saccharomyces cerevisiae was investigated by tracing the transfer of radioactive methyl groups from S-adenosyl methionine, taken up by growing cells, into the protein moiety of ribosomes. It was estimated that the large subunit contained about 10 protein-bound methyl groups distributed mainly among proteins YL23, YL32 and YL1. The small subunit contained at most 2–4 methyl groups in proteins. Methyl groups could be transferred in vitro to proteins YL23 and YL32 in extracts from cultures of an S-adenosyl methionine auxotroph deprived of the methyl-group donor. In the most heavily methylated proteins the methylated amino acids formed in vitro were the same as those found in vivo (monomethyllysine and dimethyllysine in YL32; dimethyl and trimethyllysine in YL23). It is concluded that the enzymatic reaction in vitro faithfully saturates with methyl groups the target amino acids which are normally fully methylated in vivo.