By the use of a variety of cell culture and separation methods, four cell lines were isolated from a single autochthonous BALB/cfC3H mammary tumor. These lines differ markedly from each other in culture morphology, various in vitro growth properties, expression of murine mammary tumor virus antigen, and karyotype, yet all four lines are tumorigenic in normal, syngeneic hosts, yielding tumors of generally similar histology, although distinct from the original neoplasm. Three of the four lines have been cloned from soft agar. The clones exhibit the same growth properties as the lines from which they were derived. Karyotypic analysis of the parent tumor revealed the presence of cells with heterogeneous numbers of chromosomes similar to those seen in the isolated lines, suggesting both the presence of these distinct cell types in the original neoplasm and a genetic origin of the diversity.