Ribonuclease T1 modified with diazonium-1-(H)-tetrazole (DHT-RNase T1) reported by Kasai has been known to be inactive towards RNA, but active towards guanylyl (3'→5')-cytidine (GpC). In order to explain this peculiar phenomenon, a couple of experiments were performed. DHT-RNase T1 is almost inactive towards poly G and poly C as well as RNA, but active against GpCpC. Michaelis constants of DHT-RNase T1 for GpC was about twice as large as that of native RNase T1. Although Ki value for 2', (3')-GMP of DHT-RNase T1 was about 4 times as large as that of native RNase T1, that of (Cp)3Gp for DHT-RNase T1 was more than 60 times larger than that for the native enzyme. From the results described above, it was suggested that a peculiar nature of DHT-RNase T1 towards RNA was probably due to the unfavorable effect of the modification on the binding of guanylic acid residue having polynucleotide at 5'-OH side.