Initial monoclonal antibody therapy trials include an attempt to control malignant proliferation of small cell lung cancer by blocking the autocrine stimulation of gastrin releasing peptide. A critical issue is the adequacy of antibody penetration into the tumor bed to effect immunologic blockade of the mitogenic peptide. The use of an indium-111 antibody chelate which is coadministered with the first therapeutic antibody administration facilitates analysis of the pharmacokinetic dynamics for this trial. If this approach is successful with gastrin releasing peptide, other peptide hormones with autocrine effects could also be targeted. A combination of anti growth factor therapies could lead to successful therapeutic control of this lethal disease.