Pharmacokinetics of ¹³¹I-labeled ch81C6 mAb of selected tumor sites from several patients. The initial tumor uptake reached a maximum between 40 and 100 hours among different tumor sites where the half-life of the clearance phase varied between 86 and 191 hours. The average biologic clearance half-life of the mAb in these tumors was estimated at 410 hours, demonstrating the long-term retention of the antibody in tumor sites. The range of absorbed doses varied between 363 and 1517 cGy. The measured activity concentrations from tumor biopsy samples were 70 and 36 kBq/g for patients 1 (•) and 2 (▪), respectively. The error bars indicate the error associated with the biopsy.