Choriocarcinoma developed from a complete hydatidiform mole has an unique genetic feature that involves monoallelic contribution from the paternal genome. To determine the chromosome carrying putative tumor suppressor gene(s), microcell-hybrids were isolated following fusion of choriocarcinoma cells with microcells from mouse A9 cells containing a single human chromosome (1, 2, 6, 7, 9 or 11). Microcell-hybrids with the introduction of chromosome 7 were suppressed or modulated for tumorigenicity and exhibited altered in vitro growth properties. Introduction of chromosomes 1, 2, 6, 9 or 11 had no effect. Tumorigenic revertants isolated from microcell-hybrids with the introduced chromosome 7 contains reduced numbers of chromosome 7. These findings suggest that chromosome 7 contains a putative tumor suppressor gene(s) for choriocarcinoma. Alterations in tumorigenic phenotypes seen in microcell-hybrids were not associated with the presence of either ERV3 or H-plk locus located on the introduced chromosome 7, indicating the putative tumor suppressor gene(s) is outside of ERV3 and H-plk gene loci. Furthermore, we obtained evidence to define a critical region on chromosome 7 (7p12-7q11.23) that was frequently lost in surgically removed choriocarcinoma tissues and cell lines. Using a panel of microsatellite markers, biallelic deletions were observed, which strongly suggests the presence of a tumor suppressor gene(s) within this critical region.