The implantation intraperitoneally of in vitro cell lines of the SEWA mouse ascites tumor into syngeneic mice and into allogeneic thymusless nude mice stimulated mitotic activity in the host component of the ascites cell population. The mitoses were found almost exclusively during the first month after the change in environment, while the tumor cells gradually readapted to in vivo growth. Normal mitoses appeared immediately after the transfer in vivo, reached a maximum at days 10 to 20, then decreased in frequency and disappeared completely after day 33. At that time, the tumor cells had recovered viability and started growing with full vigor. The ascites samples in which normal cells were undergoing mitotic division often contained a low proportion of tumor cells and a high proportion of small inflammatory cells. The chromosomes of the normal mitoses differed from those of the tumor mitoses by being smaller and more crowded. Their relative uniformity in type was also accentuated by the fact that they were all characterized by a low but ubiquitous incidence of chromatid breaks. These features suggested that the normal mitoses belonged to the lymphocytic population and represented a defense mechanism of the host towards the tumor. Since normal mitoses were especially frequent in the ascites cell population of the thymusless mice, it may be speculated that T-lymphocytes were not the main target cells for the mitotic induction.