Surgically resected fresh human tumors (16 malignant and 1 benign) in the head and neck regions were s.c. transplanted into scid mice. All malignant tumors (12 squamous cell carcinomas, 2 papillary adenocarcinomas and 1 adenoid cystic carcinoma) except one heavily irradiated squamous cell carcinoma could grow in scid mice. However, all of three squamous cell carcinomas of the maxillary sinus grew only in 50% of scid mice, and two of them failed to grow in the second transfer. The remainder were successfully transplantable for further generations, except one accidental case. A benign tumor, a follicular adenoma of the thyroid gland, was also accepted in scid mice and was transplantable for further generations, though its growth was very slow. Distant metastases were found in the lung only when poorly differentiated carcinomas were transplanted into scid mice, but did not occur until 90 days after the transplantation of well and moderately differentiated carcinomas. The histological characteristics of both malignant and benign tumors were retained well in all of the xenografts and metastatic lesions. Thus, scid mice seem useful to investigate not only the properties of benign and malignant human tumors but also the metastatic spread of tumors which threaten the life of cancer patients.