Seven microbial and one mammalian species of cytochrome c have been reacted with O-methylisourea to convert lysine residues to homoarginines containing enriched 13C. This set of guanidinated cytochromes has been assayed for electron transport reactivity and the nuclear magnetic resonance spectra of incorporated label have been obtained. The set consisted of c-type cytochromes from horse, Saccharomyces cerevisiae, Candida krusei, Paracoccus denitrificans, Pseudomonas aeruginosa, Rhodospirillum rubrum, Rhodopseudomonas capsulata, and Rhodopseudomanas spheroides. All derivatives demonstrated high electron transport reactivity with cytochrome oxidases; at some concentrations this rate was 100% or higher compared to corresponding native rates. All labeled ferricytochrome spectra followed a common pattern giving about five resolved or partially resolved resonance peaks. Two of these, at approximately 158.1 and 157.3 parts per million, correspond to single carbon sites. They have been assigned to labeled lysine 27 and lysine 79 (horse numbering), respectively, on the basis of sequence comparisons and an approximate chemical shift calculation. Labeled ferrocytochrome spectra were obtained and shown to be more diverse than the set of ferric spectra. Poly-[ 13C]homoarginine was prepared and shown to be an inhibitor of the horse cytochrome c-cytochrome oxidase reaction but an activator for the reactions of Paracoccus cytochrome c550. Relaxation measurements indicated that polyhomoarginine forms a complex with both cytochromes c.