Cellular immunity to tumor-associated antigens of C3H/HeN-MTV+ (hereafter called C3H+) and C3H/HeN-MTV-(hereafter called C3H-) mice bearing syngeneic transplantable mammary tumors was assessed by the production of migration inhibition factor in response to mouse mammary tumor virus (MTV)-related antigens or 3 m KCl tumor extracts, by the use of an indirect agarose microdroplet migration inhibition assay. C3H+ mice bearing a transplantable syngeneic mammary tumor (MAT-2) were generally unable to produce migration inhibition factor in response to MTV, whereas a strong immune response in those mice was elicited by a 3 m KCl extract of MAT-2 tumor. C3H-tumor-bearing mice were reactive to MTV as well as to 3 m KCl extract. The latter reactivity was found to be specific since these mice failed to react against similarly prepared 3 m KCl extracts of various normal or chemically induced tumor tissues. Two other spontaneously arising mammary tumors (MAT-3 and MAT-4) of C3H+ mice appeared to share an antigen with MAT-2. Furthermore, a 3 m KCl extract of C3H+ embryos, with no detectable MTV antigens, was found to stimulate migration inhibition reactivity in C3H+ and C3H- mice bearing MAT-2 tumor, which suggests that tumor growth induces reactivity against non-virus-related tumor-associated antigens, which are common to embryo cells. Induction of migration inhibition factor production by MAT-2 tumor-associated antigens or MTV appeared to be dependent on T-cells, since reactivity was eliminated by pretreatment of immune spleen cells with anti-Thy plus complement.