The method of in vivo absorption of xenoantisera raised against murine chemically-induced tumors has been used to obtain specific anti-tumor sera. To assess if the tumor specificity was maintained under different experimental conditions, the sera were employed in in vitro tests based on different immunologic reactions. In vivo absorption results in the production of xenoantisera which are highly specific for the immunizing tumor and that lack antibody activity toward histocompatibility and organ specific antigens and endogenous virus antigens. Thus, in vivo absorbed antisera appear to be valuable reagents for the immunochemical characterization of chemically-induced tumors.