We examined the ability of human chromosome 11 derived from normal fibroblast cells to suppress the tumorigenicity of SiHa cells, a human cervical tumor cell line. Using DNA transfection, the human chromosome was tagged with a selectable marker (the pSV2neo gene, which encodes resistance to the antibiotic G418), transferred to mouse A9 cells by cell hybridization and microcell transfer techniques, and then transferred to SiHa cells by microcell transfer. These procedures resulted in the appearance of 15 independent, G418-resistant clones, 5 of which had one or two extra copies of an intact human chromosome 11. In situ chromosomal hybridization of these clones with the pSV2neo plasmid revealed the presence of a neo-tagged human chromosome 11 in all of the five SiHa-microcell hybrids. Two SiHa-microcell hybrids that contained a single copy of neo-tagged human chromosome 12 were also isolated by the same methods. The tumorigenicities of SiHa clones with one or two extra copies of chromosome 11 (SiHa-11) were suppressed; four of the five SiHa-11 clones formed no tumors in nude mice, whereas both parental SiHa cells and SiHa cells with an extra chromosome 12 formed tumors within 30 d. One SiHa-11 cell clone formed a single tumor 90 d after injection. This rare tumor had lost one copy of chromosome 11 and rapidly formed tumors when reinjected. These results indicate that the introduction of a single copy of normal human chromosome 11, but not chromosome 12, suppresses the tumorigenicity of SiHa cells, indicating the presence on human chromosome 11 of a putative tumor-suppressor gene (or genes) for human cervical tumors.