The in vitro action of some natural polyphenolic preparations, extracted from the leaves of Asclepias syriaca, upon the proteinosynthesis of HeLa cancerous cells and, implicitly, upon the development of HeLa cells cultures was investigated. The significant perturbation of proteic biogenesis, the inhibition of HeLa tumoral cells cultures development, as well as the existence of a dose--response relationship argue the behaviour of these products as in vitro active cytostatic agents. This characterization justifies their introduction in the in vivo screening program on rats bearing of different experimental tumoral lines, for the preclinical pharmacological evaluation of the POLYAS I and POLYAS II vegetable polyphenols antineoplastic activity.