Primary human kidney and bladder tumours were implanted into both nude rats (rnu/rnu) and nude mice (nu/nu). 40% of bladder tumours and 27% of kidney tumours were established as xenografts. Histological sections of primary tumour and xenografts confirmed their similarities. Two patients with renal tissues were hypercalcaemic prior to nephrectomy, and animals in which these tumours grew as xenografts also became hypercalcaemic. Cell lines were established in tissue culture from two of the kidney tumour xenografts, and chromosome studies have confirmed their human karyotype.