Blood protein binding by the food-borne carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was investigated using male Sprague-Dawley rats. Among the many blood proteins modified in rats dosed intragastrically with [3H(G)]IQ, hemoglobin and albumin were modified in a dose dependent fashion. Albumin bound 3 - 5 times more IQ than hemoglobin at doses from 2 to 150 μmol. IQ-modified serum albumin was enzymatically digested using Pronase and analyzed by h.p.l.c. Many peptide fragments containing radioactivity were detected but the low level of protein modification (0.01 - 0.04% of dose) prevented spectroscopic analyses of these adducts. An in vitro system containing hepatic microsomes metabolized IQ to a reactive species which could bind to serum albumin. One major adduct was formed at the cysteine34 residue using this activation system and was identical to an adduct isolated from in vivo-modified albumin. Chemical and spectroscopic analyses of the Pronase fragment proved the adduct was a tripeptide containing N2-cysteinesulfinyl-IQ. A chemically identical adduct was formed in vitro when N-hydroxy-IQ was incubated with serum albumin. As much as 10% of the IQ bound to serum albumin in vivo was present as this sulfur-linked adduct based on h.p.l.c. analysis of the Pronase digest fragments and on the acid-labile activity which could be recovered as IQ.