Abstract
The study shows inhibition of the antitumor effect of cyclophosphamide (CPA) on mammary carcinomas growing in mice prior-immunized by intragastric inoculations of the tumor. Immunization with tumor cells through this route produces under certain conditions factors able to modulate the antigenic determinants on tumor cells. The possibility that the antigenicity of tumor cells might be an important factor in obtaining successful therapy with CPA is proposed.