We have assessed the antitumoral action of the POLYAS I and POLYAS II vegetal polyphenolic biopreparations--separated and purified from Asclepias syriaca leaves - in rats with various experimental tumoral lines. We studied the therapeutic effect of different doses on the tumor generation process and compared it with the experimental oncostatic action of several standard chemotherapeutic drugs of clinical use (thiotepa, methotrexate, melphalan and cyclophosphamide). In our experimental treatment with the bioactive polyphenolic agents, we have used various doses, both higher and lower than the dose that had conditioned the expression of their antitumoral action upon Guerin T-8 lymphotropic epithelioma and upon Walker 256 carcinosarcoma. We found the antineoplastic effectiveness of those aromatic biopreparations from phytomass to be dose-dependent. We compared the evaluation indices of the antitumoral pharmacodynamic effect we obtained in the treatment with the POLYAS biopreparations with those of reference cytostatic agents. The antitumoral potential of the new natural biopreparations is higher than, equal or close to that of the standard oncochemotherapeutic agents. Antitumoral effectiveness can be improved by an experimental manipulation of the therapeutic doses--which proves the existence of a dose-response relationship. POLYAS I and POLYAS II polyphenolic biopreparations are compatible in point of effectiveness with the standard cytostatic agents, a fact that we considered relevant for the characterization of the POLYAS I and POLYAS II vegetal extracts as potential antineoplastic agents. The quantitative preclinical evaluation of the specific pharmacodynamic effect will be complemented by the investigation of the new polyphenolic biopreparations therapeutic effectiveness in tumors with various degrees of development.