A variety of heterologous mammalian cells were inoculated into nude mice and scored for tumorigenicity. The cells tested were from primary cell cultures, established cell lines of neoplastic origin, established cell lines of nontumor origin and primary cell cultures transformed by oncogenic viruses. Regardless of the animal species of origin, every cell line that was tumorigenic in some other animal host and every cell line of neoplastic origin was tumorigenic in nude mice. Several tissue culture cells lines capable of indefinite growth in vitro failed to form tumors in nude mice, and the basis of their growth suppression was investigated. The failure of an established cell line to form tumors in nude mice is apparently an authentic response to host-mediated growth-regulatory signals.