Male Sprague-Dawley-rats were dosed with carbon-14 labeled ortho- toluidine (95534) (OT) at intraperitoneal doses of 10, 20, 40, 50, or 100mg/kg and sacrificed at 2, 4, 8, 18, 24, 48, or 72 hours or 7, 14, or 28 days following dosing. Peak binding for albumin (Alb) was recorded at 50mg/kg at 4 hours. The peak binding for hemoglobin (Hb) was recorded at 24 hours at the 100mg/kg dose level. OT-Hb binding appeared to increase linearly in a dose dependent manner, while the OT-Alb binding was not linear. OT bound to Alb or Hb demonstrated half lives of 2.6 and 12.3 days, respectively. A two fold increase in radioactivity bound to Hb was observed after intraperitoneal administration of 10mg/kg OT when compared to the binding observed following oral dosing. Pretreatment with phenobarbital caused an increase in OT-Hb binding. This was not noted following pretreatment with beta-naphthoflavone. The authors conclude that OT binds to both Alb and Hb and that a linear dose relationship exists for Hb. The biological half lives for the protein adducts were several times those reported for elimination of OT or its metabolites via the urine. This provides evidence that these proteins may be valuable biomarkers of exposure to OT in the occupational setting.