We performed radioimmunoscintigraphy (RIS) and / or pharmacokinetic (PCK) studies in 12 patients with primury or metastatic colorectal carcinoma, utilizing an intravenous administration of 1–4 mCi (1 mg) of1311-B72.3 monoclotial antibody. Metastatic lesions were correctly identified in 418 patients by RIS. Two patients with small lesions (>2 cm diameter) had a false-negative RIS scan. Two patients had a true-negative RIS scan. Optimal images were obtained at I week peek postinjection. PCK studies showed that the plasma clearance of131I-B72.3 was biexponetial with an alpha-phase half-life ranging from 0.5 to 7.1 hr and a beta-phase half-life ranging from 47.5 to 85.3 hr. Systemic and renal clearance data indicated that131I-B72.3 was cleared very slowly and almost entirely by deiodination.This pilot study wus conducted to gain an understanding of the pharmacokinetics of this radiolabeled antibody. On the basis of these data, we are now studying second-generation antibodies as part of our long-range objectives to incorporate them in early detection and treatment protocols.