Studies were performed to ascertain the effects of transplantation of thymic cells exposed in vivo to 3-methylcholanthrene (3-MC) on the induction of malignancies in Copenhagen rats. Three recipient rats unexpectedly developed tumors which bore histological resemblance to myositis ossificans of humans. Specifically, histology revealed areas of peripheral ossification with the appearance of zones of primitive osteoid with a central cellular area. Other areas of the lesions were less well organized into characteristic zones or were more or less heterogeneous. The primary, as well as recurring, lesions appeared in the axilla and were well circumscribed, 24-68 g in weight and 2-7 cm in diameter. Flow cytometric analyses of DNA content indicated that these tumors contained cells with abnormal DNA characteristics as well as proliferating cells. Coupled with the observation that after excision these tumors recurred, the data suggest that these myositis ossificans lesions were malignancies.