Many yeasts secrete proteins which are toxic for pathogenic and non-pathogenic microorganisms. These toxins, mostly glycoproteins, consist of membrane-binding subunits which interact with carbohydrates (e.g. 1,6--D-glucan or -mannan) on the cell wall of sensitive strains. The killing effect is presented by membrane permeation, cell lysis or inhibition of the cell cycle. It is also suggested that these killer glycoproteins, similar in structure to lectins, can mediate self-adhesion of the pathogenic microorganisms, thus stimulating their excretion from the intestines of infected mammals. It is supposed that the above interactions could be important for therapeutic applications, especially for enteric diseases. In order to fully understand the structural basis of the functions of killer glycoproteins, it is essential to characterize their glycosylation state and to determine the structure of all glycans attached to the proteins. In this paper, a strategic approach to the purification of yeast protein from complex biological mixtures is presented. The approach is structured into seven subassignments, each of which is essential for the successful isolation of a pure and biologically active yeast protein. The subassignments are: 1) decision on the use of the purified protein; 2) collecting information about the chemical, physical and biological properties of the protein; 3) establishing assays for the protein and its biological activity; 4) decision on the source of raw material; 5) development of an efficient extraction method; 6) development of a purification method; 7) establishment of optimum conditions for storage of the purified protein.