OBJECTIVES: I. Define the toxicity and determine the maximum tolerated dose of iodine I 131 humanized monoclonal antibody A33 (131I-huAb A33) in patients with advanced colorectal cancer. II. Describe pharmacokinetics and biodistribution of 131I-huAb A33 by external imaging in these patients. III. Determine the effect of human antihuman antibody response on pharmacokinetics and targeting of 131I-huAb A33 in this patient population. IV. Determine whether the dose planning methodology used here can adequately and safely be applied to routine radioimmunotherapy planning.