The results of a series of projects on the cytotoxic drug response of human tumour xenografts are compared. All were performed in one laboratory, using conventional CBA mice that were usually immunosuppressed by thymectomy, cytosine arabinoside treatment, and whole-body irradiation. Results on human tumours arising in 9 anatomical sites are included, with the main emphasis on colo-rectal, pancreas, breast, lung and testis carcinomas, also melanomas. Growth acceleration during successive passage of most of these tumour types was observed. When therapeutic response was measured by a growth-delay method there were wide differences in response to chemotherapy. Testicular teratomas and small-cell lung tumours responded well; breast tumours showed modest response; melanomas, colo-rectal tumours and non-small-cell lung tumours responded poorly. Studies of clonogenic cell survival were made in 11 xenografted tumour lines. They confirmed the range of responsiveness and tendency towards individuality of the growth delay data. Cell survival in most cases was exponentially related to drug dose. This compilation of a large amount of experimental data supports the belief that human tumour xenografts broadly maintain the level of chemotherapeutic responsiveness of the source tumours in man.