An insulin-secreting islet cell tumour of the Syrian hamster has been transplanted serially in the congenitally immune-deficient nude mouse, in order to test the potential usefulness of this mouse mutant as a graft carrier of heterologous tumours with stable differentiated phenotypes. The incidence of tumour growth was very high, and the hamster tumour retained its functional and histologic characteristics during consecutive passages in nude mice. These results show that nude mice may be useful carriers of differentiated tumours from non-inbred species including man, and for the isolation of cell lines from such tumours.