Primary amines react with 2, 4-pentanedione at pH 6-9 to form enamines, TV-alkyl-4-amino-3-penten-2-ones. The latter compounds readily regenerate the primary amine at low pH or on treatment with hydroxylamine. Guanidine and substituted guanidines react with 2, 4-pentane-dione to form TV-substituted 2-amino-4, 6-dimethylpyrimi-dines at a rate which is lower byat least a factor of 20 than the rate of reaction of 2, 4-pentanedione with primary amines. Selective modification of lysine and arginine side chains in proteins can readily be achieved with 2, 4-pentane-dione. Modification of lysine is favored by reaction at pH 7 or for short reaction times at pH 9. Selective modification of arginine is achieved by reactionwith 2, 4-pentanedione