A hybrid cell formed by the spontaneous fusion of two sister subpopulations was isolated and was found to combine clinically aggressive features of both parental cells. Subpopulation 168FAR is highly tumorigenic but does not metastasize spontaneously from a s.c. site. Subpopulation 44FTO is a variant selected for resistance to 6-thioguanine and to ouabain. It is poorly tumorigenic but spontaneously metastasizes to the lungs and livers of mice in which primary tumors do form. The two parental subpopulations are differentially sensitive to chemotherapeutic drugs; 168FAR is relatively more resistant to methotrexate, whereas 44FTO is more resistant to 5-fluorouracil. Both are about equally sensitive to melphalan. The hybrid 168FAR x 44FTO clone was isolated by growth in hypoxanthine-aminopterin-thymidine medium which also contained 3 mM ouabain; neither 168FAR nor 44FTO survive in this medium. The hybrid nature of the isolated clone was confirmed by cytogenetic analysis and determination of DNA content. The hybrid clone was highly tumorigenic, spontaneously metastasized from the subcutis, was nearly as resistant to 5-fluorouracil as the most resistant parental line, and was more resistant to both melphalan and methotrexate than either parental subpopulation. These results illustrate the role that cell fusion could play in progression by allowing the rapid assimilation of aggressive phenotypes from distinct coexisting subpopulations.