The proliferation of various tumour cells was inhibited in vivo and in vitro after application of/or incubation with xenogeneic liver tissue. The development of s.c.-implanted meth-A-sarcoma was blocked by the prophylactic injection of these preparations. In addition firmly established tumours regressed under therapy. Preparations obtained from xenogeneic organs like thymus, placenta or brain had a similar antitumor activity. A mixture of various xenogeneic tissues from different species had a much higher therapeutic efficiency. In the Meth-A-system the xenogeneic material surpassed the antineoplastic effect of rather high doses of cyclophosphamide. The preparations showed no side-effects in mice and rats. These results were supported by experiments in tissue culture. This new antitumour activity of xenogeneic tissues in vivo is interpreted as mediated by an increased host defense. The results in tissue culture however, indicate also a direct regulatory effect on cells.