The ratio of zinc and cadmium is of significance for error-free proliferation and differentiation of cells. An understanding of the role of cadmium in the etiology of cancer offers possibilities to gain a better understanding of cancer as well as possibilities to prevent, treat and/or alleviate cancer. The concentration of Zn(II) in fetal serum, human serum from umbilical cord blood or healthy donors, and in avian serum including intact eggs and egg products has significance for the reliability and repeatability of cell and tissue culture method using said serum, or serum containing products, or eggs. Disclosed is a method in the manufacture of serum products, in particular serum products intended for use in in vitro culture of mammalian cells or tissues, wherein the concentration of Zn(II) is determined, and preferably adjusted to a desired interval. Different products consisting of or containing serum or serum fractions are also disclosed.