Inoculation of adult hamsters with greater than 1.5 X 10(2) Herpesvirus type 2 (HSV-2) transformed hamster embryo fibroblasts induced rapidly growing tumours at the site of inoculation, in more than 50 per cent of animals. The tumours were examined by light and electron microscopy, and were histologically anaplastic fibrosarcomas. Lung metastases were frequently found in tumour-bearing animals; these were usually only seen microscopically, and were cytologically similar to the primary tumours. The nature of tumour development showed similarities with naturally occurring tumours of man, and HSV-2 tumour is probably a better model of human cancer than other virus-induced tumours studied in the past.