Somatic cell genetic studies gave the first proof that functional tumor suppressor genes exist in mammalian genomes. Initial studies showed that whole-cell hybrids between tumorigenic mouse or human cell lines and their normal counterparts became nontumorigenic upon inoculation into animals. However, identification of the operative tumor suppressor gene proved difficult due to the presence of the entire chromosomal complement of the normal cell parent. The development of the technique of microcell hybridization has provided a powerful method for overcoming this obstacle. Suppression of transformed properties of a cancer cell line upon transfer of single human chromosomes from normal cells directly maps the location of tumor suppressor activity. One can then use positional cloning techniques or differential expression strategies to isolate the functional tumor suppressor gene. We present a general strategy for the mapping of tumor suppressor genes in mammalian cells. We also outline some of the important control experiments as well as pitfalls encountered in such studies.