B H K 21/13 cells, 50 gcnerutioris f r o m cloning, are rransplaiitablc~ owing to the prcwiice qf a sniall proportion (about 0.00i 74) of' highly tuniorigenic variarir cells.
Noti-tuiriorigeriic cells can easily be i.solated,froni thcsr sfoclis by srcloning. With passagc in vitro the proportion of tirinorigenic variants can increase to over 90 %. This niuy be (lup to any or all of three situations: I ) thc apprasancc of many difiiwwt new tumorigenic variants, 2 ) the selection of the initially o h s e r v d variant cells because of their grraft'r capacity for growth in culture, or 3 ) thc p r e s r n c ~ of a replicating agent transinittcd from a variant to othes cdls. Meuris by which the increase in the proportion of variants might be niininiized are discussed.
I n contrast to B H K 211 I3 cells in early passagc., their polyoniatransformed dcsivatives arc highly tuniorigeriic. Polyonia virus does not selcdvely transforni existing tirinorigenic variant cc4s; thus, thc tirniorigrriicity of the transformed cdls is a co~isoqitericc of the virus-cell interaction.
BHK 21/13 is a clone of fibroblastic cells, derived from the kidneys of newborn hamsters (Macpherson and Stoker, 1962). It is widely used in studies on virus-cell interactions, particularly those which involve oncogenic viruses. When the cells of this clone are exposed to high multiplicities of polyoma virus, a small proportion of the cells is transformed (Macpherson and Stoker, 1962;Stoker and Abel, 1962). The transformed cells have many characteristics in common with the cells from turnours induced in hamsters by polyoma virus, one of which is the capacity to produce tumours when transplanted into adult hamsters (Stoker, 1962;Macpherson, 1963). This suggests that transformation may be analogous to oncogenesis in vivo, and that the tumorigenicity of the transformed cells is a result of the viruscell interact ion. However, untransforrned BHK 21/13 cells which have been grown for long periods in vitvo may be tumorigenic also (Defendi et al., 1963; Gottlieb-Stematsky and Shilo, 1964;Stoker and Macpherson, 1964). This fact questions the validity of tumorigenicity as a specific character of polyoma-transformed BHK 21/13 cells. This paper is concerned with the tumorigenicity of B H K 21/13 cells, their spontaneous variants and their polyoma-transformed derivatives. MATERIAL A N D METHODS 1. Cdls B H K 21/13 cells (subsequently referred to as C 13).
The number of generations was estimated on the basis that the mean doubling time of C13 cells