We have retrospectively investigated the response to cisplatin of a squamous cell carcinoma of the head and neck xenografted to nude mice during nine years of serial transplantation. Tumour growth rate decreased gradually. After nine years and over 100 passages, there was a sudden increase in cisplatin sensitivity. Histopathological examination showed that, of two histopathologically different subpopulations present in earlier passages, the predominant one was no longer detectable. The DNA-index did not change.