Immunological mechanisms are demonstrably of central importance in preventing the development of certain virus-associated cancers in animals and man; however, they do not appear to fulfill this role in the majority of 'spontaneous' tumours. This does not, however, necessarily indicate that spontaneous tumours lack potential target antigens for immunologically mediated destruction. Work in the field of transplantation immunology has clearly shown that certain cell types fail to elicit rejection reactions despite their possession of alloantigens. Similarly, some tumour cell types are poorly immunogenic to the point that they can grow in and kill animals despite a major histocompatibility barrier. These tumours are, however, susceptible to destruction in vivo in appropriately allo-sensitized animals. Thus, some tumours may be able to grow in autologous or syngeneic hosts because of their poor immunogenicity, despite the fact that they express potential (tumour-associated) rejection antigens. It may be possible to manipulate this situation for therapeutic purposes.