The cytotoxicity of catechols has been ascribed to covalent binding of the o-quinone oxidation products to proteins through the sulphydryl group. We have previously shown that dopaquinone can bind covalently to proteins through cysteine residues to form protein-bound cysteinyldopas. In this study, we have compared the reactivities of o-quinones, derived from tyrosinase oxidation of various catechols, with the cysteine residue in bovine serum albumin. o-Quinone forms of dopamine, norepinephrine, N-acetyldopa, N-acetyldopamine, 3,4-dihydroxyphenylacetic acid, pyrocatechol and 4-methylcatechols were much more reactive than dopaquinone, while o-quinone forms of 5-S-cysteinyldopa and epinephrine were much less reactive. The yield of protein-bound cysteinylcatechols appears to depend on a competition between the intermolecular nucleophilic reaction of sulphydryl groups in protein and the intramolecular nucleophilic reaction of an amino group in the side chain.