Methyl methacrylate film prepared from material similar to that used for prosthesis construction was implanted subcutaneously in the abdominal wall of 50 Harlan strain albino Swiss mice. As a control series, pieces of cellophane were implanted in a second group of 50 mice. Sections through the area of the implants from animals which did not develop tumors showed that the methyl methacrylate and cellophane films become encapsulated by connective tissue. The fibrotic reaction was greatest around the cellophane. Small foci of inflammatory cells were observed in a few instances adjacent to both materials. The first tumor appeared 257 days after implantation of methyl methacrylate at which time 20 animals were still living. Four more tumors subsequently appeared between 405 and 469 days. In the mice containing cellophane only one tumor developed, 400 days after implantation, at which time 7 of the original 50 animals were living. Tumors arising from both the methyl methacrylate and cellophane films showed considerable structural uniformity and were all diagnosed as fibrosarcomas.