HETEROLOGOUS transplantation of human neoplasms into the cheek pouch of the syrian hamster (Mesocricetus auratus) was first reported in 1951 (ref. 1). The uniqueness of the pouch as a site for transplantation results from the alymphatic structure and the very slow diffusion of large molecules out of the pouch area2. The large mass of loose connective tissue which comprises most of the pouch may be responsible for limiting the diffusion of transplantation antigens. Its selection as a favourable site for tumour transplantation has been based not only on its immunological responses, but also on its simplified structure, the ease with which transplants vascularize, the accuracy with which growth rates and regressions can be measured, and the fact that repeated eversion of the pouch for observation of transplants creates little or no trauma to host tissue or transplant.