The development of techniques for growing human tumours in immune-deficient mice is reviewed. In a compilation of success-rates in grafting a series of types of human tumour into a variety of recipients there is evidence for different levels of success among the tumour types but little firm information on the comparative receptivity of different hosts. Adenocarcinomas, particularly of the colon, have done well, as have melanomas and some tumours of the nervous system. Breast carcinomas, squamous cell carcinomas and various sarcomas have transplanted poorly. Human tumour xenografts have been found to maintain the histological characteristics of the parent tumour and in some cases hormone production and hormone dependance have also been maintained. Evidence is accumulating on the maintainance of therapeutic sensitivity of the tumours.