Methylation of horse heart cytochrome c has been examined in vitro with [ methyl- 14C]methanesulfonate (MMS) and [1- methyl- 14C]-1-nitrosourea (MNU) as alkylating agents. Analysis of protein hydrolyzates by an automatic amino acid analyzer indicates that, at pH 9.0 with MMS, ϵ- N-monomethyllysine is found to be the only major methylated basic amino acid. On the other hand, the identity of the predominant basic amino acid residue which is [ methyl- 14C]-labeled by MNU cannot be determined at present. Peptide mapping of chymotryptic digests of cytochrome c after reaction with MMS reveals a lack of specificity in methylation of a specific lysine residue in this hemoprotein.