This chapter discusses reactions of alkyl guanidines that have extended to the modification of arginine in proteins. Modification with malonaldehyde requires strong mineral acid and is accompanied by peptide bond cleavage. However, the reaction of 1,2-cyclohexanedione with arginine, which requires relatively strong alkali, is useful in studies of antigenicity of myoglobin. The observation that arginine could be modified near neutral pH with 2,3-butanedione opened the way for the application of various forms of the reagent in the study of arginine in antibodies, carboxypeptidase- A, bovine plasma albumin (BPA), bovine pancreatic ribonuclease A (RNaseA), α1-acid glycoprotein, and low density lipoprotein. Glyoxal and phenylglyoxal are used as mild arginine reagents. The chapter discusses the preparation of certain arginine reagents and conditions for their use. Arginine can be modified by dimeric or trimeric 2,3-butanedione to give three ninhydrin-positive final products. The 2,3-butanedione undergoes rapid self-condensation in mildly alkaline or even neutral phosphate buffer, modification of arginine employing the monomer in this buffer is extremely complex.