The histological structure of 58 primary tumours derived from spontaneously transformed tissue culture cell lines from embryonic (14-16 days), young (3-20 days) and old (28-34 months) C3H and C57 mice is described. Although the cell lines were derived from a number of different organs the tumours were similar in morphology. The pattern was mixed, with “fibrosarcomatous”, “myxoid”, “epithelioid” and giant cell areas. The tumours resemble some types of haemangiopericytoma
