In 1966 Flanagan described in detail the genetics of the nude mutation and confirmed that the lack of hair was determined by a presence of recessive, autosomal gene (named nu)(1). In 1968 Pantelouris found that this mutation was accompanied by the lack of thymus but only in the homozvgotes (2). In 1969 Rygaard and Povlsen reaorted the first successful heterotransolantation of human colon cancer-tissue to nude mice (3). These works were the milestones in the use of nude mice in oncology research. The nude mice were being widely used in oreclinical druo testing and tumor biology studies. Many human tumor cell lines or fresh tumor tissue specimens grow after transolantation into nude mice (4). However-. not all human tumors can be successfuly t. ransplanted 1nt. o these mice. A vat-ietv of tumor traits and host depended factors have been defined which can influence the lack of growth of some human tumors in immunodeficient recipients. There is a need to elucidate the mechanisms underlying the observed differences in the transolantabilitv of human tumor materials. One of the host related+ actors which is involved in determinina of the growth of heterotransolanted human tumors is an immunologic response mediated mainlv bv various cells exertina cytotoxic activitv. In this chaotet-we will review the data on cytotoxic cells which can be identified in nude mice and which may influence the growth of hrterotransplanted human tumors. A better understandinq of crytotoxic mechanisms can oDen new and promising approaches to exnerimental canter therapy. in which immunode+ icient mice xenoorafted with human tumors are being widely amplied.