The amino acid sequence of leek cytochrome c was determined with 0.4mumol of protein. The sequence was deduced solely from a chymotryptic digest. The cytochrome was homologous with other plant cytochromes c of mitochondrial origin. Leek cytochrome c has an N-acetylated ;tail' as compared with mammalian cytochrome c, and two residues of epsilon-N-trimethyl-lysine. Unlike other plant cytochromes c, leek cytochrome c has glutamic acid or glutamine in position 11, leucine in position 20 and alanine in position 51. 4-Hydroxyproline partially substitutes for proline in position 79. Experimental details are given in a supplementary paper that has been deposited as Supplementary Publication SUP 50012, at the National Lending Library for Science and Technology, Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.