Neuropeptide receptors can be used as molecular targets to deliver chemotherapeutic drugs into cancer cells. The gastrin-releasing peptide (GRP) receptor was used to deliver a camptothecin (CPT)–bombesin (BB) conjugate into the lung cancer cell line NCI-H1299. The CPT–BB conjugate was metabolized by NCI-H1299 intracellular enzymes releasing the cytotoxic CPT. Receptor binding methods and fluorescent methods are presented for studying the internalization of GRP receptors. The MTT, clonogenic, and 3H-thymidine uptake assays are presented for studying the proliferation of NCI-H1299 cells. The results indicate that CPT-BB conjugates are internalized as a result of receptor-mediated endocytosis.