The induction of neoplasms in mice inoculated shortly after birth with cell-free extracts or filtrates prepared from mouse tumors has been reported by several investigators (1-9). In this laboratory, pleomorphic tumors of the parotid glands were commonly produced with such materials. Cell-free extracts prepared from the parotid-gland tumors did not induce neoplasms in mice that were inoculated within 24 hours after birth. We were able, however, to demonstrate the release of a tumor-inducing substance from such neoplasms by the use of tissue culture (10). Monkeykidney tissue cultures were inoculated with mouse leukemia or parotidgland tumor preparations and incubated at 36 C. for 1 to 2 weeks. A tumor-inducing agent which could be passed serially in vitro was demonstrated in the supernatant fluids from 8 of 28 such tumor preparations. In the work reported here, tissue cultures prepared from minced, mouse embryo cells were found to be more effective than monkey kidney cells for the propagation of the tumor-producing agent as shown by: 1) a greater frequency of certain primary tumors in the injected mice; 2) a shortening of the latent period or the time required for appearance of the tumors after inoculation; 3) successive passage of five sublines of the agent in tissue culture, one of which is now in its 29th passage; 4) tumor-inducing activity of the agent at dilutions of 10-5 and occasionally at 10-7•