Olipifat is an antineoplastic drug containing pyrophosphate and a product of special lignin processing. Donor C57Bl/6J mice with syngeneic B16 melanoma received a single 5-day course of olipifat. Effect of olipifat on antitumor resistance was evaluated by local neutralization test [3]. In animals with rapid melanoma growth, splenic cells from intact donors stimulated tumor growth. Olipifat abolished this growth-stimulating effect of splenocytes. In animals with slow melanoma growth, splenocytes had no effect on the growth of melanoma or Lewis lung cancer. In this case, splenocytes from olipifat-treated donors completely arrested the growth of melanoma B16 and decelerated the growth of Lewis lung carcinoma.