[Two-stage carcinogenesis induced by foreign bodies].

Replacement of non-perforated polyvinylchloride films 3.5 months after implantation to CBA mice by perforated polyvinylchloride films or by millipore filters with 0.45 micron pores led to the development of sarcoma at the site of implantation in 37.0 and 21.4% of cases, respectively. The implantation of only perforated films led to the development of sarcoma in 6.9% of cases, while the implantation of filters failed to induce tumour growth. The data obtained show that the latent period of tumour development can be divided into two stages. The first stage occurs in the presence of non-perforated film, while the second stage ensues in the presence of non-cancerogenic films. Complete removal of films 6 months after implantation prevented tumour development. It is suggested that non-perforated films serve as initiating agents, while non-cancerogenic films act as protectors.