In 32 experiments involving transplantation of human malignant melanomas to the mouse mutant nude, tumourtake was observed in 14 instances. The tumour tissue which took and grew after inoculation was derived from malignant melanoma metastases of the skin (10 takes from 13 transplantations), and from primary malignant melanomas of the eye (4 takes from 6 transplantations). Not one of 13 attempts to transplant primary tumours of the skin was successful. In 7 instances serial transfers of transplanted tumour were successful, the degree of success ranging from 3 to 41 passages. Some passages were wilfully interrupted but two are continuing. The transplanted tumours grow locally. Neither lymph node metastases nor organ metastases have been observed. Histological appearances of the transplanted tumours, also after serial passages, were identical with those of the original human donor material. The human tumour/nude mouse system is contrasted and compared with other heterotransplantation models.