In the present article we describe studies on a chemically induced sarcoma in DA rats. This tumor expresses a unique antigen which can be demonstrated by both syngeneic antibodies and cytotoxic T cells. We have established cytotoxic T-cell lines (CTLs) specific for the tumor and with high efficient killing capacity in vitro. When testing for the ability of such CTLs to inhibit tumor out-growth in vivo, we found that they had to be inoculated together with the tumor and in the presence of T-cell growth factor to provide any significant degree of protection. We thus believe not only that there is a requirement for addition of CTL-stimulating lymphokines in vivo, but also that the CTLs fail to move from one site in vivo to attack relevant tumor cells at another site. No evidence was obtained that the CTLs gradually could acquire such migratory ability in vivo.