Spontaneous thymoma in an inbred strain of rat.

Thymomas that appear with high frequency in inbred WAB rats living into old age were examined. The spontaneous thymomas described were discrete, encapsulated bodies composed of lymphocytes and epithelial cells, with a predominance of lymphocytes. The rat neoplasms resembled the human thymomas, and their rate of incidence in older animals may suggest their use as models for the human condition.