Abstract: Extracts which were recovered with aqueous extraction media from the pollen of plants inhibit the growth of tumor-cells and stimulate at the same time the metabolism of normal healthy cells. Said extracts of pollen which do not contain more than 5% by weight of proteins having a high molecular weight or which are completely free of such proteins of high molecular weight, are used as active ingredient of corresponding pharmaceutical preparations and they are used for treating cancer, tumors and leukemia in human beings or warm blooded animals. Living leukemia cells are rapidly killed by said extracts. The pollen extracts in question are a yellowish-white product and they usually contain, referred to the dry weight of the pollen extracts, at least 70% by weight of carbohydrates, 5-12% by weight of amino acids, peptides and/or proteinous substances and furthermore water soluble vitamines of the group B and mineral constituents.