Although more than 90% of the radioactivity was excreted in the urine and feces within 10 days of oral treatment of sheep with a single dose of p [14C]toluoyl chloride phenylhydrazone (TCPH), persistent blood residues (5–6 ppm) were observed for at least 21 days. The14C residues were largely localized in erythrocytes and covalently bound to both heme and globin. Only the phenyl group of the phenylhydrazlne part of TCPH was present as14C bound residues. An analytical procedure to measure the level of phenyl groups incorporated in heme, based on their oxidation to benzoic acid, was developed to monitor residues in treated animals. Relay metabolism in rats was studied by feeding sheep blood containing14C residues form [14C] TCPH treatment. No retention of14C residues in rat tissues was observed, which contrasted with the TCPH metabolism. A 90‐day relay toxicity study in rats, which were fed dried blood from treated sheep containing up to 2,000 times the potential exposure to residues in the human diet, indicated no observable toxic responses. It is concluded that these data support a tolerance of 6 ppm TCPH equivalents in blood.