After nude-mouse-passage, a carcinoma of the nasopharynx was found to contain a few scattered C-type retrovirus particles. Culture of this nude-mouse-grown material over solid agar allowed the human tumour cells to grow as multicellular spheroids which increased in number by budding and could be subcultured for up to 6 months. Within the spheroids the human tumour cells expressed increased retrovirus replication and large numbers of C-type particles were observed. Treatment with a halogenated pyrimidine further enhanced the virus production. This tissue culture system provided sufficient virus production for the identification of the retrovirus as an endogenous xenotropic murine leukaemia virus and not a human agent.