7-Methyl-9-ethyl-guanine ((7m)G) was titrated by eleven cytosine-containing dipeptides (C-Xxx) monitoring the chemical shift of H8 proton. Amino acids incorporated to C-Xxx were related to the molecular ratio and association constant of the complex. In the Eadie-Hoffstee plots, the dipeptides of which side chains could form hydrogen bonds showed two phases. The highest association constant was observed for C-Asn, and its level was 20 times greater than that of C-Trp.