1, 2-Cyclohexanedione reacts specifically with the guanidino group of arginine or arginine residues at pH 8 to 9 in sodium borate buffer in the temperature range of 25-40 degrees. The single product, N-7, N-8-(1,2-dihydroxycyclohex-1,2-ylene)-L-arginine (DHCH-arginine) is stable in acidic solutions and in borate buffers (pH 8 to 9). DHCH-Arginine is converted to N-7-adipyl-L-arginine by periodate oxidation. The structures of the two compounds were elucidated by chemical and physicochemical means. Arginine or arginyl residues can be regenerated quantitatively from DHCH-arginine by incubation at 37 degrees in hydroxylamine buffer at pH 7.0 FOR 7 TO 8 hours. Analysis of native egg white lysozyme and native as well as oxidized bovine pancreatic RNase, which were treated with cyclohexanedione, showed that only arginine residues were modified. The utility of the method in sequence studies was shown on oxidized bovine pancreatic ribonuclease A. Arginine modification was complete in 2 hours at 35 degrees in borate buffer at pH 9.0 with a 15-fold molar excess of the reagent. The derived peptides showed that tryptic hydrolysis was entirely limited to peptide bonds involving lysine residues, as shown both by two-dimensional peptide patterns and by isolation of the resulting peptides. The stability of DHCH-arginyl residues permits isolation of labeled peptides.